Parasitic channel mitigation in III-nitride material semiconductor structures

ABSTRACT

III-nitride materials are generally described herein, including material structures comprising III-nitride material regions and silicon-containing substrates. Certain embodiments are related to gallium nitride materials and material structures comprising gallium nitride material regions and silicon-containing substrates.

TECHNICAL FIELD

III-nitride materials are generally described, including gallium nitridematerials and associated material structures including substratescomprising silicon.

BACKGROUND

III-nitride materials include gallium nitride (GaN), aluminum nitride(AlN), indium nitride (InN) and their respective alloys (e.g., AlGaN,InGaN, AlInGaN and AlInN). In particular, gallium nitride materialsinclude gallium nitride (GaN) and its alloys such as aluminum galliumnitride (AlGaN), indium gallium nitride (InGaN), and aluminum indiumgallium nitride (AlInGaN). These materials are semiconductor compoundsthat have a relatively wide, direct bandgap which permits highlyenergetic electronic transitions to occur. Such electronic transitionscan result in gallium nitride materials having a number of attractiveproperties including the ability to efficiently emit blue light, theability to transmit signals at high frequency, and others.

In many applications, III-nitride materials are typically grownheteroepitaxially on a substrate. However, property differences betweenIII-nitride materials (e.g., gallium nitride materials) and manysubstrate materials can present challenges. For example, gallium nitridematerials (e.g., GaN) have a different thermal expansion coefficient(i.e., thermal expansion rate) and lattice constant than many substratematerials and, in particular, silicon. These differences may lead toformation of cracks and/or other types of defects in gallium nitridematerial layers that are grown heteroepitaxially on silicon. In somemethods, a transition layer is used to mitigate the effects of thesedifferences in order to grow high quality gallium nitride material onsilicon. However, these differences (and others) have limited theperformance and commercialization of structures and devices that includegallium nitride material formed on silicon substrates.

III-nitride materials (e.g., gallium nitride materials) are beinginvestigated in high frequency (e.g., RF and power management) deviceapplications. When energy is dissipated in high frequency devicesthrough undesirable mechanisms (e.g., parasitic losses and capacitivecoupling), the performance of the device may be impaired. Theseso-called parasitic losses can reduce output power, switching speed,power gain, and efficiency. Therefore, it is generally desirable tolimit the parasitic losses in high frequency (and other types of) RF andpower management devices.

SUMMARY

III-nitride materials are generally described herein, including materialstructures comprising III-nitride material regions andsilicon-containing substrates. Certain embodiments are related togallium nitride materials and material structures comprising galliumnitride material regions and silicon-containing substrates. The subjectmatter of the present invention involves, in some cases, interrelatedproducts, alternative solutions to a particular problem, and/or aplurality of different uses of one or more systems and/or articles.

Certain embodiments are related to semiconductor structures. Someembodiments are related to methods of forming semiconductor structures.

According to certain embodiments, the semiconductor structure comprisesa substrate comprising silicon, and a III-nitride material regionlocated over a surface region of the substrate, wherein the surfaceregion of the substrate comprises a low-conductivity parasitic channelor the substrate is free of a parasitic channel, and at least a regionof the substrate comprises at least one species having a relative atomicmass of less than 5 at a concentration of at least about 10¹⁹/cm³.

In some embodiments, the method of forming a semiconductor structurecomprises implanting a species having a relative atomic mass of lessthan 5 into a substrate comprising silicon to produce a surface regioncomprising no parasitic channel or comprising a low-conductivityparasitic channel wherein, during the implanting step, at least aportion of the species is implanted through a III-nitride materialregion.

The method of forming a semiconductor structure comprises, according tosome embodiments, implanting a species having a relative atomic mass ofless than 5 into a structure comprising a III-nitride material regionand a substrate comprising silicon, wherein at least a portion of thespecies is implanted through the substrate without being implantedthrough the III-nitride material region, and implanting the speciesproduces a surface region comprising no parasitic channel or alow-conductivity parasitic channel.

In certain embodiments, the semiconductor structure comprises asubstrate comprising silicon, and a III-nitride material region locatedover a surface region of the substrate, wherein the substrate comprisesat least one p-type dopant defining a p-type dopant concentrationprofile, and the substrate comprises at least one n-type dopant definingan n-type dopant concentration profile that is substantially matched tothe p-type dopant concentration profile.

The method of forming a semiconductor structure comprises, according tocertain embodiments, implanting a counter-dopant into a semiconductorstructure comprising a III-nitride material region and a substratecomprising silicon such that a concentration profile of thecounter-dopant substantially matches a concentration profile of a seconddopant present within the substrate.

The semiconductor structure comprises, according to some embodiments, asubstrate comprising silicon and at least one active species coupledwith an external species or capable of reacting with an externalspecies, and a III-nitride material region located over a surface regionof the substrate, wherein the concentration of the active species is atleast about 10¹⁹/cm³.

In certain embodiments, the method of forming a semiconductor structurecomprises forming a III-nitride material region over a surface region ofa substrate comprising silicon such that a species within the substratereacts with at least a portion of an external species that contacts thesubstrate during the formation of the III-nitride material region.

According to some embodiments, the semiconductor structure comprises asubstrate comprising silicon and comprising at least a layer having aresistivity of greater than 10² Ohms-cm; a low-temperature AlN regionlocated over the substrate; a high-temperature AlN region located overthe substrate; and a III-nitride material region located over thelow-temperature AlN region and over the high-temperature AlN region.

The method of forming a semiconductor structure comprises, in certainembodiments, forming a first AlN region over a substrate comprisingsilicon and comprising at least a layer having a resistivity of greaterthan 10² Ohms-cm, wherein the temperature of the environment in whichthe first AlN region is formed is between about 700° C. and about 950°C.; forming a second AlN region over the substrate, wherein thetemperature of the environment in which the second AlN region is formedis from about 950° C. to about 1150° C.; and forming a III-nitridematerial region over the first AlN region and over the second AlNregion.

The semiconductor structure comprises, according to some embodiments, asubstrate comprising silicon and comprising at least a layer having aresistivity of greater than 10² Ohms-cm; a diffusion barrier regioncomprising a rare-earth oxide and/or a rare-earth nitride located over asurface of the substrate; and a III-nitride material region located overthe diffusion barrier region comprising the rare-earth oxide and/or therare-earth nitride.

The method of forming a semiconductor structure comprises, according tocertain embodiments, forming a diffusion barrier region comprising arare-earth oxide and/or a rare-earth nitride over a substrate comprisingsilicon, the substrate comprising at least a layer having a resistivityof greater than 10² Ohms-cm; and forming a III-nitride material regionover the diffusion barrier region comprising the rare-earth oxide and/orthe rare-earth nitride.

In certain embodiments, the semiconductor structure comprises asubstrate comprising silicon and comprising at least a layer having aresistivity of greater than 10² Ohms-cm; a diffusion barrier regioncomprising silicon carbide located over a surface of the substrate; anda III-nitride material region located over the diffusion barrier regioncomprising silicon carbide.

In some embodiments, the method of forming a semiconductor structurecomprises forming a diffusion barrier region comprising silicon carbideover a substrate comprising silicon, the substrate comprising at least alayer having a resistivity of greater than 10² Ohms-cm; and forming aIII-nitride material region over the diffusion barrier region comprisingsilicon carbide.

The semiconductor structure comprises, according to some embodiments, asubstrate comprising silicon and comprising at least a layer having aresistivity of greater than 10² Ohms-cm; a diffusion barrier regioncomprising an elemental diboride located over a surface of thesubstrate; and a III-nitride material region located over the diffusionbarrier region.

In certain embodiments, the method of forming a semiconductor structurecomprises forming a diffusion barrier region comprising an elementaldiboride over a substrate comprising silicon, the substrate comprisingat least a layer having a resistivity of greater than 10² Ohms-cm; andforming a III-nitride material region over the diffusion barrier region.

According to certain embodiments, the semiconductor structure comprisesa substrate comprising silicon; a III-nitride material region locatedover a surface region of the substrate; and an implanted speciesarranged within the surface region of the substrate in a patternspatially defined across at least one lateral dimension of thesubstrate, wherein the implanted species is present within at least aportion of the surface region of the substrate at a concentration of atleast about 10¹⁹/cm³.

The method of forming a semiconductor structure comprises, according tosome embodiments, implanting a species into a surface region of asubstrate comprising silicon such that the implanted species forms apattern spatially defined across at least one lateral dimension of thesubstrate, wherein, during the implanting step, at least a portion ofthe species is implanted through a III-nitride material region, andafter the implanting step, the implanted species is present within atleast a portion of the surface region of the substrate at aconcentration of at least about 10¹⁹/cm³.

According to some embodiments, the semiconductor structure comprises asubstrate comprising silicon; a III-nitride material region located overa surface region of the substrate; a first implanted species arrangedwithin the surface region of the substrate in a first pattern spatiallydefined across at least one lateral dimension of the substrate; and asecond implanted species arranged within the III-nitride material regionin a second pattern spatially defined across at least one lateraldimension of the substrate.

The method of forming a semiconductor structure comprises, according tosome embodiments, implanting a first species and a second species into asemiconductor structure comprising a substrate comprising silicon and aIII-nitride material region located over the substrate, wherein thefirst species is implanted into a surface region of the substrate suchthat the first species forms a pattern spatially defined across at leastone lateral dimension of the substrate; and the second species isimplanted into the III-nitride material region such that the secondspecies forms a pattern spatially defined across at least one lateraldimension of the III-nitride material region.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1A is a cross-sectional schematic illustration of a semiconductorstructure, according to certain embodiments;

FIG. 1B is a cross-sectional schematic illustration of a semiconductorstructure comprising an optional diffusion barrier region, according tocertain embodiments;

FIG. 1C is, according to some embodiments, a cross-sectional schematicillustration of a multi-layered diffusion barrier region;

FIG. 1D is a cross-sectional schematic illustration of a multi-layeredIII-nitride material region, according to some embodiments;

FIG. 1E is a cross-sectional schematic illustration of a multi-layeredIII-nitride material device region, according to certain embodiments;

FIG. 2A is, according to some embodiments, a cross-sectional schematicillustration of a semiconductor structure comprising a transistor;

FIG. 2B is a cross-sectional schematic illustration of a semiconductorstructure comprising an optional diffusion barrier region and atransistor, according to certain embodiments;

FIG. 2C is, according to certain embodiments, a top-view schematicillustration of a semiconductor structure comprising a transistor;

FIG. 2D is a side-view, cross-sectional schematic illustration of thesemiconductor structure illustrated in FIG. 2C;

FIG. 2E is a side-view, cross-sectional schematic illustration of aportion of the semiconductor structure illustrated in FIG. 2C;

FIG. 3A is, according to some embodiments, a cross-sectional schematicillustration showing the transport of species into a semiconductorstructure;

FIG. 3B is a cross-sectional schematic illustration showing thetransport of species into a semiconductor structure comprising multiplelayers within a III-nitride material region, according to certainembodiments;

FIG. 3C is, according to certain embodiments, a top-view schematicillustration of a semiconductor structure comprising a transistor,including an implantation mask;

FIG. 3D is a side-view, cross-sectional schematic illustration of thesemiconductor structure illustrated in FIG. 3C;

FIG. 3E is a side-view, cross-sectional schematic illustration of aportion of the semiconductor structure illustrated in FIG. 3C;

FIG. 3F is, according to certain embodiments, a top-view schematicillustration of a semiconductor structure comprising a transistor,including an implantation mask;

FIG. 3G is a side-view, cross-sectional schematic illustration of thesemiconductor structure illustrated in FIG. 3F;

FIG. 3H is a side-view, cross-sectional schematic illustration of aportion of the semiconductor structure illustrated in FIG. 3F;

FIG. 3I is, according to certain embodiments, a top-view schematicillustration of a semiconductor structure comprising multipletransistors;

FIG. 3J is a side-view, cross-sectional illustration of thesemiconductor structure illustrated in FIG. 3I;

FIG. 3K is a schematic illustration of an implantation mask, accordingto certain embodiments;

FIG. 3L is a schematic illustration of an implantation mask, accordingto certain embodiments;

FIG. 3M is a schematic illustration of an implantation mask, accordingto certain embodiments;

FIG. 4A is, according to some embodiments, a cross-sectional schematicillustration showing the transport of species into a back side of asubstrate of a semiconductor structure;

FIG. 4B is a cross-sectional schematic illustration showing thetransport of species into a back side of a substrate of and into asemiconductor structure comprising multiple layers within a III-nitridematerial region, according to certain embodiments;

FIG. 5 is an exemplary plot of free carrier concentration as a functionof depth in a substrate, according to certain embodiments;

FIG. 6 is a plot of free carrier concentration as a function of depthinto a substrate, including examples in which protons (H⁺) wereimplanted into a substrate and examples in which protons (H⁺) were notimplanted into a substrate; and

FIG. 7 is a plot of free carrier concentration as a function of depthinto a substrate, including examples in which oxygen (O⁺) was implantedinto a substrate and examples in which oxygen (O⁺) was not implantedinto a substrate.

DETAILED DESCRIPTION

III-nitride materials are generally described herein, including materialstructures comprising III-nitride material regions andsilicon-containing substrates. Certain embodiments are related togallium nitride materials, and material structures comprising galliumnitride material regions and silicon-containing substrates.

Some embodiments are related to mitigating the effect of parasiticchannels (also sometimes referred to as parasitic conducting channels)present within semiconductor structures and/or inhibiting or preventingthe formation of parasitic channels within semiconductor structures.Parasitic channels can arise when growing III-nitride materials (e.g.,gallium nitride materials) on substrates comprising silicon, andparticularly on substrates comprising silicon that are highly resistive(e.g., having an electrical resistivity of greater than or equal toabout 10² Ohm-cm). It is believed that the parasitic channel is formedby the diffusion of dopants that are unintentionally introduced into thesubstrate prior to, or during, the growth of layers/regions (e.g.,III-nitride material region) on the substrate. As described furtherbelow, the dopants may include the elements (e.g., Group III element(s)and/or nitrogen) from the reactive species which participate in thereaction that forms the III-nitride material region. For example, thedopants may be gallium and/or aluminum; though, it should be understoodthat other dopants may also contribute to forming the parasitic channel.Once diffused into the substrate, the dopants can generate free carriers(i.e., electrons or holes) which, in effect, form a conductive channelat or near the substrate surface.

Parasitic channels are typically formed in a surface region (e.g., at ornear a surface of the substrate over which the III-nitride material isgrown also referred to herein as the “top” surface of the substrate). A“surface region” of a substrate can include an external surface of asubstrate and a portion of the substrate underneath and close to theexternal surface. In some embodiments, the surface region of a substrateextends to a depth of about 5 microns, to a depth of about 2 microns, toa depth of about 1 micron, to a depth of about 500 nm, or to a depth ofabout 200 nm, or less. The “top surface region” is the surface regionassociated with the top surface of a substrate. In any instances inwhich a surface region of a substrate is recited herein, the surfaceregion may correspond to the top surface region of the substrate.According to certain embodiments, the surface region of the substrate isall or part of a silicon layer.

According to certain embodiments, semiconductor structures comprising aparasitic channel (e.g., a high-conductivity parasitic channel) can beprocessed such that the adverse impact of an existing parasitic channelis reduced or eliminated. The adverse impact of an existing parasiticchannel can be reduced or eliminated, for example, by reducing the freecarrier concentration in the parasitic channel. In some suchembodiments, parasitic losses and/or capacitive coupling associated withundesired conduction through the parasitic channel are reduced orminimized. In some embodiments, one or more species can be implantedinto a semiconductor structure such that the electronic conductivity ofthe high-conductivity parasitic channel within the semiconductorstructure is reduced (e.g., such that it becomes a low-conductivityparasitic channel or such that the parasitic channel is eliminated).Reduction of the conductivity of the high-conductivity parasitic channelmay be achieved, for example, by implanting a species (e.g., asmall-atom species) that disrupts the structure of the high-conductivityparasitic channel. For example, the implanted species may disrupt thecrystalline lattice structure of the substrate and/or thehigh-conductivity parasitic channel. The disruption of the crystallinelattice structure of the substrate and/or the high-conductivityparasitic channel may reduce the ability of the dopants to contribute toconduction (e.g., by reducing the mobility of the dopants). Reduction ofthe conductivity of the parasitic channel can also be achieved, forexample, by implanting one or more counter dopants into thesemiconductor structure such that the concentration profile of thecounter dopant substantially matches the concentration profile of thedopant within the high-conductivity parasitic channel. In this way, thecontribution of the original dopant to the electronic conductivity ofthe high-conductivity parasitic channel can be at least partiallynegated by the counter dopant.

Some embodiments relate to methods (and associated structures) that canbe used to inhibit or prevent the diffusion of material that increasesthe conductivity of the substrate, which can lead to the formation ofhigh-conductivity parasitic channels. For example, in some embodiments,one or more species capable of reacting with a species external to thesemiconductor structure are located within a substrate of thesemiconductor structure, and a III-nitride material region is formedover the substrate. During the formation of the III-nitride materialregion, the reactive species within the substrate can react with adiffusing Group III element(s), nitrogen, or other species originatingfrom outside the semiconductor structure, which can, according tocertain embodiments, inhibit or prevent the formation of ahigh-conductivity parasitic channel. The diffusion of material can alsobe inhibited or prevented, for example, via the use of one or morelayers positioned between the III-nitride material region and thesubstrate. The one or more layers positioned between the III-nitridematerial region and the substrate may act, in some embodiments, as adiffusion barrier. According to certain embodiments, careful selectionof an appropriate diffusion barrier material is considered, to allow forheteroepitaxial formation of high quality gallium nitride materialand/or gallium nitride material microelectronic and/or optoelectronicdevice layers.

As described further below, mitigating the effects of parasitic channelscan be desirable because parasitic channels can be particularlysignificant in leading to parasitic losses in semiconductor structures,including semiconductor structures formed on highly resistive substrates(e.g., highly resistive silicon substrates). The parasitic channels canprovide a mechanism for undesired energy absorption within thestructure. When processing methods are used to mitigate the effects ofparasitic channels present in semiconductor structures and/or to inhibitor prevent the formation of such parasitic channels, the parasiticlosses in the resulting semiconductor structures may be significantlyreduced which can result in performance improvements. As onenon-limiting example, certain devices (such as High Electron MobilityTransistors (HEMTs) and RF devices) formed of certain structuresdescribed herein may have higher output power, power gain, and/orefficiency, (even at higher operating frequencies), amongst otheradvantages. In yet another non-limiting example, certain power switchingdevices may exhibit a lower Rds(on) shift and/or degradation.

Certain embodiments are related to inventive semiconductor structures.Certain inventive semiconductor structures can comprise a substrate(e.g., a substrate comprising silicon) and a III-nitride material regionlocated over a surface region of the substrate. FIG. 1A is across-sectional schematic illustration of a semiconductor structure100A, according to certain embodiments. Semiconductor structure 100Acomprises substrate 110 and a III-nitride material region 120 locatedover surface 135 of substrate 110 (and, thus, over surface region 130 ofsubstrate 110). According to certain embodiments, surface 135 ofsubstrate 110 can be a silicon surface. For example, surface 135 maycorrespond to a surface of a bulk silicon wafer, in some embodiments. Incertain embodiments, the top surface (e.g., surface 135 in the figures)may correspond to a silicon surface of a composite substrate (e.g.,comprising a silicon layer and one or more additional underlyinglayers). For example, in some embodiments, surface 135 may correspond toa surface of a silicon portion of a silicon-on-insulator substrate,surface 135 may correspond to a surface of a silicon-on-sapphiresubstrate, or surface 135 may correspond to a silicon surface of aseparation by implantation of oxygen (SIMOX) substrate.

According to certain embodiments, the substrates of the semiconductorstructures described herein comprise silicon (i.e., a substratecontaining the element silicon in any form). Examples of substratescomprising silicon that can be used in various embodiments include, butare not limited to, silicon carbide substrates, bulk silicon wafers, andsilicon on insulator substrates. In some embodiments, the substratecomprises a silicon substrate. As used herein, a silicon substraterefers to any substrate that includes a silicon surface. Examples ofsuitable silicon substrates include substrates that are composedentirely of silicon (e.g., bulk silicon wafers), silicon-on-insulator(SOI) substrates, silicon-on-sapphire substrate (SOS), and separation byimplantation of oxygen (SIMOX) substrates, amongst others. Suitablesilicon substrates also include composite substrates that have a siliconwafer bonded to another material such as diamond or othercrystallographic forms of carbon, aluminum nitride (AlN), siliconcarbide (SiC), or other crystalline or polycrystalline materials.Silicon substrates having different crystallographic orientations may beused, though single crystal silicon substrates may be preferred incertain, but not necessarily all, embodiments. In some embodiments,silicon (111) substrates are used. In certain embodiments, silicon (100)or (110) substrates are used.

As used herein, a silicon carbide substrate refers to any substrate thatincludes a silicon carbide surface. Examples of suitable silicon carbidesubstrates include substrates that are composed entirely of siliconcarbide (e.g., bulk silicon carbide wafers), silicon carbide compositewafers (e.g., wafers comprising a silicon carbide layer and a secondlayer of a material that is not silicon carbide), and the like.

In certain embodiments, the substrate may have various device layers,homojunctions, heterojunctions, or circuit layers embedded in thesubstrate, or formed on the front-side or back-side of the substrate.Such substrates may be semi-spec standard thickness, or thicker, or insome implementations thinner than semi-spec standards. In some cases,for example, the Si substrate may have a diameter of less than onehundred millimeters (100 mm), while in other implementations, thesubstrate may have a diameter in a range from approximately 100 mm toapproximately 150 mm. In certain embodiments, the substrate diameter maybe in a range from approximately 150 mm to approximately 200 mm, orlarger. In still other embodiments, the substrate may include a texturedsurface or may have a non-planar surface. The substrate may also haveany of a variety of suitable thicknesses. For example, in someembodiments, the substrate has a thickness greater than or equal toabout 250 micrometers, greater than or equal to about 500 micrometers,greater than or equal to about 625 micrometers, greater than or equal toabout 675, greater than or equal to about 1 mm, or thicker (e.g., havinga thickness of up to about 2 mm, up to about 3 mm, up to about 5 mm, upto about 10 mm, or thicker). In some embodiments, the substrate has athickness of less than about 10 mm, less than about 5 mm, less thanabout 3 mm, less than about 2 mm, less than about 1 mm, less than about500 microns, less than about 200 microns, less than about 150 microns,less than about 100 microns, less than about 50 microns, or less.According to certain embodiments, the thickness of the substrate may beselected based on the final device and heteroepitaxial specifications(e.g., wafer warp and bow), for example, as needed for successful highyielding semiconductor fabrication.

In some embodiments, the substrate comprises at least a layer having ahigh resistivity. For example, in certain embodiments in which siliconsubstrates are used, the silicon substrate (or at least the siliconportion of the substrate for substrates that include a silicon portionformed on another material) is highly resistive. According to certainembodiments, the substrate comprises at least a layer having aresistivity of greater than or equal to about 10² Ohms-cm (or greaterthan or equal to about 10⁴ Ohms-cm, or greater than or equal to about10⁵ Ohms-cm). For example, in certain embodiments in which siliconsubstrates are used, the resistivity of the silicon substrate (or thesilicon portion of the substrate) may be greater than or equal to about10² Ohms-cm (or greater than or equal to about 10⁴ Ohms-cm, or greaterthan or equal to about 10⁵ Ohms-cm). In certain embodiments, theresistivity of the surface region of the substrate may be greater thanor equal to about 10² Ohms-cm (or greater than or equal to about 10⁴Ohms-cm, or greater than or equal to about 10⁵ Ohms-cm). Highlyresistive substrates comprising silicon (e.g., silicon substrates orother substrates comprising silicon) may be particularly useful in some(but not necessarily all) structures that are used to form devices thatoperate at high frequencies (e.g., RF devices). According to certainembodiments, the high resistivity can reduce so-called substrate losseswhich otherwise may arise and sacrifice performance. These substratelosses may render substrates comprising silicon with lower resistivitiesunsuitable in high frequency devices.

It has been observed that parasitic channels generally have asignificantly greater effect in structures that include highly resistivesubstrates comprising silicon as compared to structures that includesubstrates having more conventional resistivities (e.g., 0.01-0.1Ohms-cm). Because substrates comprising silicon having more conventionalresistivities typically have bulk free carrier concentrations on theorder of 10¹⁸/cm³, the dopant diffusion phenomena described above maynot substantially, or even at all, change the free carrier concentrationat the substrate surface. Thus, a parasitic channel may not be generatedin such substrates. In contrast, the dopant diffusion phenomena can havea significant effect on the free carrier concentration at the surfaceregion in substrates having high resistivities which typically have bulkfree carrier concentrations on the order of 10¹⁴/cm³ or lower. For thesereasons, it is generally more critical to reduce the effects of theparasitic channel in structures that include highly resistive substratescomprising silicon.

As used herein, the term “III-nitride material” refers to any Group IIIelement-nitride compound. Non-limiting examples of III-nitride materialsinclude boron nitride (BN), aluminum nitride (AlN), gallium nitride(GaN), indium nitride (InN), and thallium nitride (TIN), as well as anyalloys including Group III elements and Group V elements (e.g.,Al_(x)Ga_((1-x))N, Al_(x)In_(y)Ga_((1-x-y))N, In_(y)Ga_((1-y))N,Al_(x)In_((1-x))N, GaAs_(a)P_(b)N_((1-a-b)),Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b)), and the like).Typically, when present, arsenic and/or phosphorus are at lowconcentrations (e.g., less than 5 weight percent). III-nitride materialsmay be doped n-type or p-type, or may be intrinsic. III-nitridematerials may have any polarity including but not limited to Ga-polar,N-polar, semi-polar, or non-polar crystal orientations. A III-nitridematerial may also include either the Wurtzite, Zincblende, or mixedpolytypes, and may include monocrystalline, polycrystalline, oramorphous structures.

In some embodiments, the III-nitride material region comprises a galliumnitride material, as described further below. As used herein, the phrase“gallium nitride material” refers to gallium nitride (GaN) and any ofits alloys, such as aluminum gallium nitride (Al_(x)Ga_((1-x))N), indiumgallium nitride (In_(y)Ga_((1-y))N), aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N), gallium arsenide phosphoride nitride(GaAs_(a)P_(b)N_((1-a-b))), aluminum indium gallium arsenide phosphoridenitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), amongstothers. Typically, when present, arsenic and/or phosphorus are at lowconcentrations (i.e., less than 5 weight percent). In certainembodiments, the gallium nitride material has a high concentration ofgallium and includes little or no amounts of aluminum and/or indium. Inhigh gallium concentration embodiments, the sum of (x+y) may be lessthan 0.4, less than 0.2, less than 0.1, or even less. In some cases, itis preferable for the gallium nitride material layer to have acomposition of GaN (i.e., x+y=0). Gallium nitride materials may be dopedn-type or p-type, or may be intrinsic.

When a structure (e.g., layer and/or device) is referred to as being“on,” “over,” or “overlying” another structure (e.g., layer orsubstrate), it can be directly on the structure, or an interveningstructure (e.g., a layer, air gap) also may be present. A structure thatis “directly on” or “in direct contact with” another structure meansthat no intervening structure is present. It should also be understoodthat when a structure is referred to as being “on” or “over” anotherstructure, it may cover the entire structure, or a portion of thestructure.

In certain embodiments, the substrate and the III-nitride materialregion can be in direct contact, as illustrated in FIG. 1A. In some suchembodiments, a silicon portion of the substrate and the III-nitridematerial region are in direct contact. Inventive semiconductorstructures are not limited to arrangements in which the substrate andthe III-nitride material region are in direct contact, and in someembodiments, one or more regions may be positioned between the substrateand the III-nitride material region. For example, in some embodiments, adiffusion barrier region is positioned between the substrate and theIII-nitride material region. FIG. 1B is a cross-sectional schematicillustration of a semiconductor structure 100B, according to certainembodiments. In FIG. 1B, semiconductor structure 100B comprises optionaldiffusion barrier region 140 positioned between substrate 110 andIII-nitride material region 120.

In some embodiments in which the diffusion barrier region is present,the diffusion barrier region is in direct contact with the substrate(e.g., a silicon surface of the substrate). For example, in theillustrative embodiment shown in FIG. 1B, diffusion barrier region 140is in direct contact with substrate 110. In other embodiments, one ormore layers may be positioned between the diffusion barrier region andthe substrate (e.g., between the diffusion barrier region and a siliconsurface of the substrate).

In certain embodiments, the diffusion barrier region is in directcontact with the III-nitride material region. For example, in theillustrative embodiment shown in FIG. 1B, diffusion barrier region 140is in direct contact with III-nitride material region 120. In otherembodiments, one or more layers may be positioned between the diffusionbarrier region and the III-nitride material region.

In some embodiments, the diffusion barrier region may be formed prior tothe introduction into the reaction chamber of reactive species (e.g.,Al, Ga species) that react to form the III-nitride material region. Insome such embodiments, the diffusion barrier region limits, or prevents,dopant accumulation on the substrate surface, amongst other functions.Because the dopant concentration accumulated on the substrate surface isreduced, dopant diffusion into the substrate is also decreased, whichcan result in the parasitic conducting channel (e.g., within the surfaceregion of the substrate) having a lower conductivity. It should beunderstood, however, that in some cases, some dopant diffusion may occurthrough the diffusion barrier region and into the substrate.

The diffusion barrier region may be formed of a number of materials, asdescribed in more detail below. In some embodiments, the diffusionbarrier region has an amorphous (i.e., non-crystalline) crystalstructure. In some embodiments, the diffusion barrier region may have asingle crystal or poly-crystalline structure.

In some embodiments, the diffusion barrier region may be very thin. Forexample, according to certain embodiments, the diffusion barrier regionhas a thickness of less than about 500 nm, less than about 200 nm, lessthan about 100 nm, less than about 50 nm, less than about 20 nm, or lessthan about 10 nm (and/or, in some embodiments, as thin as 5 nm, as thinas 1 nm, or thinner). Thicker diffusion barrier regions could also beused. For example, in some embodiments, the diffusion barrier region hasa thickness of less than about 5 microns, less than about 4 microns,less than about 3 microns, less than about 2 microns, or less than about1 micron. In some embodiments, the diffusion barrier region has athickness of at least about 10 nm or at least about 100 nm.

In some, but not necessarily all, embodiments, it may be beneficial toemploy a very thin diffusion barrier region when the diffusion barrierregion is amorphous. Not wishing to be bound by any particular theory,very thin layers formed directly on the substrate may absorb strainassociated with lattice and thermal expansion differences between thesubstrate and overlying layers/regions (e.g., III-nitride materialregion). This absorption of strain may reduce generation of misfitdislocations (and other types of defects) and limit/prevent crackgeneration in the overlying layers/regions. In certain although notnecessarily all embodiments, it may be beneficial to use a thindiffusion barrier if the diffusion barrier is made of a poor thermalconductor. In some such cases, using a thin diffusion barrier layer canminimize the adverse effect on thermal conduction of the finalIII-nitride material-based device.

In the embodiment illustrated in FIG. 1B, diffusion barrier region 140covers substantially the entire top surface 135 of substrate 110. Thisarrangement may be preferable, in certain but not necessarily allembodiments, for example, to minimize dopant diffusion and thegeneration of dislocations in overlying regions. In other embodiments,the diffusion barrier region does not completely cover the top surfaceof the substrate. In some embodiments, the layer may cover a majority ofthe top surface of the substrate (e.g., greater than about 50 percent orgreater than about 75 percent of the top surface area).

It should be understood that the term “region” may refer to one layer ormay refer to multiple layers. As one non-limiting example, the diffusionbarrier region may be made of a single layer or it may comprise aplurality of layers. FIG. 1C is a cross-sectional schematic illustrationof a diffusion barrier region 140 comprising multiple layers 140A and140B. In one set of embodiments, layers 140A and 140B correspond to ahigh-temperature AlN layer and a low-temperature AlN layer, as describedin more detail below. Multi-layer diffusion barrier region 140illustrated in FIG. 1C can be used in association with any of theembodiments described herein. Alternatively, as noted above, thediffusion barrier regions described herein may be single-layerstructures.

Similarly, the III-nitride material region may be made of a single layeror it may comprise a plurality of layers. In some cases, the III-nitridematerial region may also comprise a non-III-nitride material layer orfeature. FIG. 1D is a cross-sectional schematic illustration of aIII-nitride material region 120 comprising multiple layers. Multi-layerIII-nitride material region 120 illustrated in FIG. 1D can be used inassociation with any of the embodiments described herein. Alternatively,as noted above, the III-nitride material regions described herein may besingle-layer structures.

In certain embodiments, the III-nitride material region comprises anoptional III-nitride material nucleation layer. For example, referringto the exemplary embodiment of FIG. 1D, III-nitride material region 120comprises III-nitride material nucleation layer 155. It should beunderstood that nucleation layer 155 is optional, and in otherembodiments, III-nitride material region 120 does not include nucleationlayer 155.

The nucleation layer can, according to certain embodiments, prepare asurface of the substrate for growth of III-nitride material over thesubstrate. In certain cases, III-nitride material (e.g., gallium nitridematerials and/or other III-nitride materials) can be difficult to growheteroepitaxially directly on the substrate (and/or another region thatis over a surface of the substrate), for example, because theIII-nitride material one wishes to grow may have a lattice structureand/or a lattice constant which is significantly different than thesubstrate or other underlying region. According to certain embodiments,the nucleation layer forms an appropriate template to transition fromthe lattice of the substrate (or other underlying layer) to a templatemore suitable for III-nitride growth. In certain embodiments, thenucleation layer can accommodate the difference in the lattice constantsof an overlying layer in the III-nitride material region (e.g., theIII-nitride material region portion in direct contact with thenucleation layer) and the region underneath the nucleation layer (e.g.,the substrate and/or another underlying region, which in some cases, maybe in direct contact with the nucleation layer). In some embodiments,the nucleation layer can accommodate the difference in the thermalexpansion coefficients of an overlying layer in the III-nitride materialregion (e.g., the III-nitride material region portion in direct contactwith the nucleation layer) and the region underneath the nucleationlayer (e.g., the substrate and/or another underlying region, which insome cases, may be in direct contact with the nucleation layer).According to certain embodiments, the nucleation layer can accommodateboth the difference in lattice constants and the difference in thermalexpansion coefficients of an overlying layer in the III-nitride materialregion and the region underneath the nucleation layer.

According to certain embodiments, the nucleation layer comprises analuminum nitride material. As used herein, the phrase “aluminum nitridematerial” refers to aluminum nitride (AlN) and any of its alloys, suchas aluminum gallium nitride (Al_((1-x))Ga_((x))N), aluminum indiumnitride (Al_((1-x))In_((x))N), aluminum indium gallium nitride(Al_((1-x-y))In_((x))Ga_((y))N), aluminum indium gallium arsenidephosphoride nitride (Al_((1-x-y))In_(x)Ga_(y)As_(a)P_(b)N_((1-a-b))),amongst others. In certain embodiments, the aluminum nitride materialhas a high concentration of aluminum and includes little or no amountsof gallium and/or indium. In high aluminum concentration embodiments,the sum of (x+y) may be less than 0.4, less than 0.2, less than 0.1, oreven less. In some cases, it is preferable for the aluminum nitridematerial to have a composition of AlN (i.e., x+y=0). Aluminum nitridematerials may be doped n-type or p-type, or may be intrinsic. In certainembodiments, the use of an aluminum nitride material as the nucleationlayer may be preferred in certain cases in which the III-nitridematerial is formed on the substrate without the use of a diffusionbarrier region between the III-nitride material and the substrate. Incertain embodiments, it may be preferred to forego the use of anucleation layer if a diffusion barrier region between the substrate andthe III-nitride material region is employed that comprises a latticetype and/or a lattice constant that is more suitable or favorable forthe growth of the III-nitride material region. Non-limiting examples ofsuch diffusion barrier regions include, but are not limited to, aluminumnitride diffusion barrier regions, rare-earth oxide and/or rare-earthnitride diffusion barrier regions, silicon carbide diffusion barrierregions, and elemental diboride diffusion barrier regions, examples ofwhich are described in more detail below.

According to certain embodiments, the nucleation layer may comprise oneor more layers. When multiple nucleation layers are present, thenucleation layers may be made of the same material or differentmaterials. In addition, in certain embodiments in which multiplenucleation layers are present, the nucleation layers may be formed usingdifferent semiconductor growth conditions. For example, in someembodiments, the nucleation layers may comprise two or more aluminumnitride material layers formed at different growth temperatures (e.g.,one at a relatively low temperature and another at a relatively hightemperature). In some embodiments, other growth conditions (e.g.,pressure, reactant flow rates, etc.) may be varied from the growth ofone nucleation layer to another.

Suitable materials from which the III-nitride material nucleation layermay be formed include, but are not limited to, aluminum nitridematerials (e.g., aluminum nitride, aluminum nitride alloys) and galliumnitride materials. The III-nitride material nucleation layer typicallyhas a constant composition.

In some embodiments, the nucleation layer comprises a first aluminumnitride-based layer and a second aluminum nitride-based layer. Thealuminum nitride-based nucleation layer can include aluminum nitride aswell as other optional elements, such as silicon and/or oxygen. Forexample, in some embodiments, the aluminum nitride-based nucleationlayer can be a thin (e.g., from about 10 Angstroms to about 20Angstroms, or thinner) amorphous or non-crystalline (unordered) materialcomprising aluminum and nitrogen, and optionally silicon and/or oxygen.In some embodiments, the amorphous aluminum nitride-based layer may alsoact as a diffusion barrier region as discussed further below. In otherembodiments, one or more separate diffusion barrier layers can be usedin combination with the aluminum-nitride based layer.

In certain embodiments, a III-nitride material nucleation layer has asingle crystal structure. It may be advantageous, in some but notnecessarily all embodiments, for a III-nitride material nucleation layerto have a single crystal structure because such structures canfacilitate formation of one or more single crystal layers (e.g., galliumnitride material layers) above the III-nitride material nucleationlayer.

It should also be understood that a III-nitride material nucleationlayer may not have a single crystal structure and may be amorphous orpolycrystalline, though certain of the advantages associated with thesingle crystal nucleation layers may not be achieved in some suchembodiments.

The III-nitride material nucleation layer may have any suitablethickness. For example, the III-nitride material nucleation layer mayhave a thickness of between about 10 nanometers and about 5 microns,though other thicknesses are also possible. In certain embodiments inwhich more than one nucleation layer is employed, the combined thicknessof the nucleation layers may be between about 10 nanometers and about 5microns, though other thicknesses are also possible.

In certain embodiments, the III-nitride material region comprises anoptional III-nitride material transition layer. For example, referringto the exemplary embodiment of FIG. 1D, III-nitride material region 120comprises III-nitride material transition layer 170. It should beunderstood that transition layer 170 is optional, and in otherembodiments, III-nitride material region 120 does not include transitionlayer 170.

In FIG. 1D, transition layer 170 is formed directly on nucleation layer155. In other embodiments, one or more materials may be positionedbetween transition layer 170 and nucleation layer 155.

In some embodiments, the III-nitride material transition layer comprisesa compositionally graded III-nitride material. Examples of suchmaterials are described, for example, in U.S. Pat. No. 6,649,287, issuedNov. 18, 2003, and entitled “Gallium Nitride Materials and Methods,”which is incorporated herein by reference in its entirety for allpurposes. Compositionally-graded transition layers have a compositionthat is varied across at least a portion of the layer (e.g., across atleast a portion of the thickness of the layer). For example, accordingto certain embodiments in which the transition layer comprises aIII-nitride material layer, the concentration of at least one of theelements (e.g., Ga, Al, In) of the III-nitride material is varied acrossat least a portion of the thickness of the transition layer.Compositionally-graded transition layers are particularly effective,according to certain embodiments, in reducing crack formation in galliumnitride material regions formed on the transition layer, for example, bylowering thermal stresses that result from differences in thermalexpansion rates between the gallium nitride material and the substrate(e.g., silicon). Compositionally-graded transition layers may alsocontribute to reducing generation of screw dislocations in theIII-nitride material layer(s)/region(s) (e.g., gallium nitride materiallayer(s)). In some cases, the compositionally-graded transition layersmay also contribute to reducing mixed and edge dislocation densities.

The composition of a compositionally-graded III-nitride material layercan be graded, for example, discontinuously (e.g., step-wise) orcontinuously. The composition of the compositionally-graded layer can begraded across the entire thickness of the layer, or across only aportion of the thickness of the layer.

According to one set of embodiments, the transition layer iscompositionally-graded and formed of an alloy of gallium nitride such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. Insome such embodiments, the concentration of at least one of the elements(e.g., Ga, Al, In) of the alloy is varied across at least a portion ofthe thickness of the transition layer. In certain embodiments in whichthe transition layer has an Al_(x)In_(y)Ga_((1-y))N composition, xand/or y may be varied. In certain embodiments in which the transitionlayer has a Al_(x)Ga_((1-x))N composition, x may be varied. In certainembodiments in which the transition layer has a In_(y)Ga_((1-y))Ncomposition, y may be varied.

In certain embodiments, it is desirable for the transition layer to havea low gallium concentration at a back surface which is graded to a highgallium concentration at a front surface. It has been found that suchtransition layers can be particularly effective in relieving internalstresses within overlying gallium nitride material layers. For example,the transition layer may have a composition of Al_(x)Ga_((1-x))N, wherex is decreased from the back surface to the front surface of thetransition layer (e.g., x is decreased from a value of 1 at the backsurface of the transition layer to a value of 0 at the front surface ofthe transition layer).

In some embodiments, the semiconductor structure includes an aluminumnitride nucleation layer and a compositionally-graded transition layer.In some embodiments, the compositionally-graded transition layer has acomposition of Al_(x)Ga_((1-x))N, where x is continuously graded from avalue of 1 at the back surface of the transition layer to a value of 0at the front surface of the transition layer. One discontinuous grademay include steps of AlN, Al_(0.6)Ga_(0.4)N, and Al_(0.3)Ga_(0.7)N (stepgrades) proceeding in a direction toward the gallium nitride materiallayer. In another example of a discontinuously graded III-nitridematerial transition layer, there may be periodic layers and/orintervening layers inserted between one or more of the step layersmaking up the step grade. The periodic layers and/or intervening layers,for example, may be layers of aluminum nitride material (e.g., AlN orAlGaN) formed at the same or different (e.g., lower) temperatures thanare used to form the step grade layers. Another example of periodiclayers or intervening layers include silicon nitride and/or aluminumsilicon nitride layers, which can act as masking layers to pin thevertical threading and screw dislocations which may extend from onelayer to the next.

It should be understood that, in other cases, the transition layer mayhave a constant composition and may not be compositionally-graded. Insome cases (e.g., in certain cases in which the substrate is not asilicon substrate and/or in certain cases in which a diffusion barrierlayer is positioned between the substrate and the III-nitride materialregion), the transition layer may have a constant composition. Suitablecompositions include, but are not limited to, aluminum nitride-basedmaterials (e.g., aluminum nitride, aluminum nitride alloys) and galliumnitride materials. In these constant composition embodiments, thetransition layer may be similar to the nucleation layer described above.In certain embodiments which utilize a diffusion barrier, the constantcomposition of the III-nitride transition layer may have a latticeconstant that approximates the lattice constant of the diffusionbarrier.

According to certain embodiments, the transition layer may be made of,at least in part, one or more superlattices (including strained layersuperlattices (SLS) or multiple quantum wells (MQW)) and/or acompositionally-graded superlattice or compositionally graded MQW.

In certain embodiments, the III-nitride material region comprises anoptional III-nitride material buffer layer. For example, referring tothe exemplary embodiment of FIG. 1D, III-nitride material region 120comprises III-nitride material buffer layer 180. It should be understoodthat buffer layer 180 is optional, and in other embodiments, III-nitridematerial region 120 does not include buffer layer 180.

The buffer layer can, according to certain embodiments, provide asurface for the growth of epitaxial III-nitride material above thebuffer layer.

According to certain embodiments, the buffer layer comprises an aluminumgallium nitride material. In some such embodiments, the buffer layercomprises Al_(x)Ga_((1-x))N. In certain embodiments in which the bufferlayer comprises Al_(x)Ga_((1-x))N, x may be less than about 0.2, lessthan about 0.1, less than about 0.05, or less than about 0.01. In someembodiments, the buffer layer comprises GaN.

The buffer layer may be formed over the transition layer, for example,using any of a number of known growth techniques. For example, accordingto certain embodiments, the buffer layer may be formed over thetransition layer using molecular-beam epitaxy (MBE) or metalorganicvapor phase epitaxy (MOVPE). In certain embodiments (including certainembodiments in which the desired epitaxial material structure will beused in the fabrication of transistors such as field effect transistors(FETs) and/or High Electron Mobility Transistors (HEMTs)), it may bedesirable to form a channel layer above the buffer layer. Typically, insome such embodiments, it would then be desirable for the buffer layerbandgap to be greater than or equal to the bandgap of the channel layer.For example, in certain transistor designs using back barriers, thechannel layer may comprise gallium nitride while the buffer layer maycomprise aluminum gallium nitride. In some such cases, the buffer layermay comprise of a substantially uniform composition of AlGaN with abandgap larger than the channel layer but smaller than the underlyingtransition layer alloy composition. In other examples, the buffer layermay itself be a compositionally graded layer which has a larger aluminumcomposition formed near the transition layer, and a smaller aluminumconcentration formed near the channel layer.

In some embodiments, the composition of the channel layer and the bufferlayer may be substantially the same. Although the intrinsic materialproperties of GaN materials can allow for the formation of highperformance devices in theory, conventional growth environments of GaNnitride materials typically include impurity sources. For example,carbon impurities resulting from metalorganic precursors may be, in somecases, introduced into the GaN materials grown using MOCVD, also knownmore generically as organometallic vapor phase epitaxy (OMVPE). Thepresence of these impurities in the GaN growth environment can causeunintentional doping in or near critical device layers, for example thechannel layer. In certain embodiments (e.g., including certainembodiments in which enhancing the standoff voltage is desired), it maybe desirable to incorporate impurities into the buffer layer. Forexample, the addition of impurities such as carbon (C) and iron (Fe)into the buffer layers of gallium nitride based transistors may increasethe vertical and lateral breakdown voltage capability and/or reduce theleakage levels of the device. However, addition of certain impuritieswithin close proximity to the channel layer may result in dispersivedevice performance (e.g., exhibited by high levels of drain and gatelag). As such, in certain embodiments, it may be beneficial to form thebuffer layer such that there is a substantially low impurityconcentration throughout the thickness of the buffer layer. In certainother embodiments, there may be a graded (continuously ordiscontinuously) impurity level within the buffer layer, with a higherimpurity concentration near the transition layer, and a lower impurityconcentration formed nearer to the channel layer. In certain otherembodiments, the transition layer(s) may also include one or moreimpurities. In some such embodiments, the concentration of impurities inthe transition layer(s) may be higher than the concentration(s) of theimpurities within the buffer layer and/or within the channel layer.

The III-nitride material region comprises, according to certainembodiments, an optional III-nitride material device region. Forexample, referring to the exemplary embodiment of FIG. 1D, III-nitridematerial region 120 comprises III-nitride material device region 190. Itshould be understood that device region 190 is optional, and in otherembodiments, III-nitride material region 120 does not include deviceregion 190.

In some embodiments, the III-nitride material region includes at leastone gallium nitride material layer. For example, in certain embodiments,the III-nitride material device region comprises at least one galliumnitride material layer. Referring to FIG. 1D, for example, in someembodiments, III-nitride material device region 190 can include at leastone gallium nitride material layer. As described further below,oftentimes, the structure includes more than one gallium nitridematerial layer which form, in part, the active region of the device.

As noted above, in some embodiments, the device region comprises one ormore III-nitride material layers. FIG. 1E is a cross-sectional schematicillustration of a III-nitride material device region 190, according tocertain embodiments. In some embodiments, the III-nitride materialdevice region comprises an optional back barrier layer. According tocertain embodiments, when present, the optional back barrier layer isthe layer of the III-nitride material region that is the closest to theunderlying substrate. For example, in FIG. 1E, exemplary III-nitridematerial device region 190 comprises optional back barrier layer 190A,which as shown in FIG. 1E, is the closest of the layers withinIII-nitride material region to substrate 110. When used, the optionalback barrier can create a double heterostructure (e.g., due to thebandgap off sets with the buffer layer, such as a GaN buffer layer).This may be desirable, in certain cases, in devices which operate underhigher drain bias as they can prevent injection of electrons from thechannel layer into the buffer layer, thereby reducing drain leakage andpunch through of the device. Additionally, in some cases, the bufferlayer may contain higher levels of impurities, intentionally (e.g., ironand carbon used to increase breakdown voltage) or unintentionally (e.g.,carbon impurities incorporated into the buffer layer as byproducts fromthe crystal growth methodologies employed). These impurities and/orother defects may, in certain cases, act as trapping centers and causedeleterious performance results (e.g., memory effects) for the device ifelectrons from the channel layer penetrate into the underlying bufferlayer. Back barrier layers can, in certain cases, help confine theelectrons in the channel layer and prevent spill over into the moredefective buffer layer and/or other underlying III-nitride layers. Incertain embodiments, one or more AlGaN back barrier layers may be used.In certain embodiments, one or more InGaN back barrier layers may beused. In some embodiments, one or more AlInN back barrier layers may beemployed. According to certain embodiments, the thickness of theback-barrier (either in the form of a single layer or a combination oflayers) is in the range of about 1-300 angstroms.

In some embodiments, the III-nitride material device region comprises anoptional channel layer. The channel layer may be positioned, accordingto certain embodiments, over the back barrier layer when present. Forexample, in FIG. 1E, exemplary III-nitride material device region 190comprises optional channel layer 190B, which as shown in FIG. 1E, ispositioned over optional back barrier layer 190A. In other embodimentsin which the back barrier layer is not present, the channel layer can bethe layer within the III-nitride material device region that is closestto the underlying substrate. According to certain embodiments, thechannel layer composition is selected with a smaller bandgap than eitherthe spacer and or front-barrier layers (described in more detail below).Such arrangements can create a heterostructure forming a two-dimensionalelectron gas (2DEG) near the interface between the channel layer and anoverlying layer (e.g., the spacer layer and/or the front barrier layer,described in more detail below). Such arrangements may be present, forexample, in High Electron Mobility Transistors (HEMTs). Electron flowthrough HEMTs and HFETs between the source and the drain of the devicecan, in some instances, be controlled by the gate of the device whichacts to interrupt electron current flow between the source and drain.The channel layer can be formed, in certain cases, such that impuritiesor other point defects (which can act as trapping centers) are kept at arelatively low level, for example, to avoid impeding the mobility of theelectrons and/or to avoid adding memory effects to the device. Trappingcenters can adversely impact linearity in RF devices and turn-on andturn-off (switching) speeds in power management devices. The thicknessof the channel layer can vary, for example, depending on the operationalvoltage desired for the device. As the drain voltage is increased, thedepth of the depletion area between the gate and drain generallyincreases. In certain cases, if the channel is formed too thin, punchthrough into the buffer layer can occur, which can result in drainleakage and breakdown of the device.

According to certain embodiments, the III-nitride material device regioncomprises an optional spacer layer (sometimes also referred to as aninterlayer). The spacer layer may be positioned, according to certainembodiments, over the channel layer and/or the back barrier layer whenpresent. For example, in FIG. 1E, exemplary III-nitride material deviceregion 190 comprises optional spacer layer 190C, which as shown in FIG.1E, is positioned over optional back barrier layer 190A and overoptional channel layer 190B. In some embodiments, the interface betweenthe channel layer and the spacer layer can form a 2-dimensional electrongas region (i.e., a “2DEG region”). For example, in FIG. 1E, 2DEG region191 is located at the interface of spacer layer 190C and channel layer190B. Typically the spacer layer, when used, is formed with a highaluminum content. In some embodiments, the spacer layer is configured tohave a relatively high bandgap offset with the underlying channel layer(e.g., by using a relatively high aluminum content in the spacer layer),which can lead to enhancement of the 2DEG. In certain embodiments, thespacer layer comprises Al_(x)Ga_((1-x))N. In certain such embodiments,(1−x)=0.5 or greater. In some embodiments, the spacer layer comprisesAlN. According to certain embodiments, the spacer layer is relativelythin (for example, less than about 50 Angstroms, less than about 20Angstroms, or less). The use of a relatively thin spacer layer canavoid, in some cases, adversely impacting the ohmic contact resistanceof the source and drain to the 2DEG and channel layer of the device.

The III-nitride material device region comprises, in some embodiments,an optional front barrier layer. The front barrier layer may bepositioned, according to certain embodiments, over the spacer layer, thechannel layer, and/or the back barrier layer when present. For example,in FIG. 1E, exemplary III-nitride material device region 190 comprisesoptional front barrier layer 190D, which as shown in FIG. 1E, ispositioned over optional back barrier layer 190A, over optional channellayer 190B, and over optional spacer layer 190C. According to certainembodiments (and as described above), if the device structure is a HEMTand/or if a 2DEG is desired, the optional front-barrier can be formedover the channel (or spacer, if used) to form a heterojunction Thecomposition of the front-barrier is selected, according to certainembodiments, such that the carrier density and/or sheet charge of the2DEG is tailored (e.g., optimized) for the device desired. In certainembodiments, the front-barrier layer comprises AlGaN. According tocertain embodiments in which an AlGaN-containing front-barrier layer isused, the aluminum concentration of the AlGaN front-barrier is less thanabout 30 atomic percent (at %), less than about 25 at %, or betweenabout 15 at % and about 20 at %. In certain embodiments, it may bedesirable to match or substantially match the lattice constant betweenthe channel and the front-barrier, and to maintain a bandgap offset, forexample, to create a 2DEG. In some such cases, an AlInN or InGaNfront-barrier layer can be created to provide the bandgap offset andmatch or substantially match the lattice constants.

According to some embodiments, the III-nitride material device regioncomprises an optional cap layer. The cap layer may be positioned,according to certain embodiments, over the front barrier layer, thespacer layer, the channel layer, and/or the back barrier layer whenpresent. For example, in FIG. 1E, exemplary III-nitride material deviceregion 190 comprises optional cap layer 190E, which as shown in FIG. 1E,is positioned over optional back barrier layer 190A, over optionalchannel layer 190B, over optional spacer layer 190C, and over optionalfront barrier layer 190D. Cap layers have been found useful inoptimizing the semiconductor surface of the device structure, accordingto certain embodiments. For example, in certain cases in which the caplayer(s) comprises GaN, the resulting surface morphology may be smootherand/or include fewer defects, relative to surfaces formed when the caplayer(s) is not present. In addition, in some cases, a more uniformsource composition may also be provided (i.e., terminated with galliumatoms rather than a mixture of gallium and aluminum atoms), which may,in some instances, aid in surface chemistry processing of thesemiconductor surface and/or reduce the number of surface defects underthe gate of the HFET. Such surface defects may, for example, act asshallow trapping centers and compromise the performance of the device,for example, due to increased gate leakage or lateral breakdown of thedevice, increased dispersion, increased gate and drain lag of thedevice, amongst other reductions in performance. Additionally, incertain cases, by providing a more consistently terminated semiconductorsurface, the repeatability and consistency of the contact resistance maybe enhanced, which can lead to higher manufacturing yields. In someembodiments, it may be desired to dope the cap layer(s). In someinstances in which the barrier is doped, it may be desirable to use aGaN cap layer which is n-type doped (e.g., using silicon). The use of ann-type doped cap layer may, according to certain embodiments, reduceIdss degradation of the device. In some embodiments, the cap layer(s)may comprise a conductive GaN material layer, for example, used as aconductive field plate.

In certain embodiments, the cap layer(s) may be doped p-type, forexample, with magnesium. Such doping may be desirable, for example, incertain cases in which a normally OFF or enhancement mode HFET is beingfabricated. As one non-limiting example, by forming a localized p-typeGaN region under the gate of the HFET, the 2DEG can be disrupted and thechannel depleted such that under no bias, the device is normally off. Insome such cases, a positive bias to the gate would then be needed torestore the 2DEG locally under the gate and allow current flow from thesource to the drain. It should be noted that one or more layers may beused as the cap layer(s), and that whether a single cap layer ormultiple cap layers are employed may depend, for example, on thespecifics of the design device structure.

In some embodiments, the cap layer may include an in-situ siliconnitride cap layer and/or passivation layer. Such layer(s) may be used toterminate the III-Nitride structure and/or stabilize the surface of theGaN material.

In some embodiments, the III-nitride material device region comprises atleast three device layers. In some embodiments, the first device layercan be the closest of the three layers to the substrate, the seconddevice layer can be an intermediate layer (i.e., between the firstdevice layer and a third device layer), and the third device layer canbe the farthest of the three layers from the substrate. In some suchembodiments, the first layer can be a channel layer, the second devicelayer can be a front-barrier layer, and the third device layer can be acap layer. Referring to FIG. 1E, for example, III-nitride materialdevice region 190 can comprise channel layer 190B, front-barrier layer190D, and cap layer 190E. In some such embodiments, back-barrier layer190A and spacer layer 190C are each optional, and one or both may bepresent or not present. According to some such embodiments, channellayer 190B can be the closest of these three layers (i.e., channel layer190B, front barrier layer 190D, and cap layer 190E) to the underlyingsubstrate. In certain embodiments, it may be preferable for the seconddevice layer to have an aluminum concentration that is greater than theconcentration of aluminum in the first layer and/or the third layer. Forexample, referring to FIG. 1E, in some embodiments, front-barrier layer190D can have an aluminum concentration that is greater than theconcentration of aluminum in channel layer 190B and cap layer 190E. Insome embodiments, the first and second layers (e.g., channel layer 190Band front-barrier layer 190D in FIG. 1E, respectively) can be galliumnitride material layers, and the value of x (as used elsewhere hereinwith reference to gallium nitride material layers, in subscripts todenote the relative amount of aluminum in a compound (e.g., the “x” inAl_(x)Ga_((1-x))) in the second gallium nitride material layer may havea value that is between about 0.15 and about 0.3 greater, or betweenabout 0.15 and about 0.75 greater than the value of x in the firstgallium nitride material layer. For example, the second device layer maybe formed of Al_(0.26)Ga_(0.74)N, while the first device layer may beformed of GaN. This difference in aluminum concentration may lead toformation of a highly conductive region at the interface of the secondand first device layer (i.e., a 2DEG region). In some embodiments, thefirst device layer may be formed of GaN.

According to certain embodiments, the III-nitride material device region(e.g., which may comprise at least one gallium nitride material layer)has a low crack level. As described above, the transition layer(particularly when compositionally-graded) and/or the nucleation layermay reduce crack formation. Gallium nitride materials and otherIII-nitride materials having low crack levels have been described, forexample, in U.S. Pat. No. 6,649,287, which is incorporated herein byreference in its entirety for all purposes. In some cases, theIII-nitride material device region (e.g., which may comprise at leastone gallium nitride material layer) has a crack level of less than 0.005μm/μm². In some embodiments, the III-nitride material device region(e.g., which may comprise at least one gallium nitride material layer)has a very low crack level of less than 0.001 μm/μm². In certain cases,it may be preferable for the III-nitride material device region (e.g.,which may comprise at least one gallium nitride material layer) to besubstantially crack-free as defined by a crack level of less than 0.0001μm/μm².

In certain cases, the III-nitride material device region (e.g., whichmay comprise at least one gallium nitride material layer) has a singlecrystal (i.e., monocrystalline) structure. In some cases, theIII-nitride material device region (e.g., which may comprise at leastone gallium nitride material layer) includes one or more layers having aWurtzite (hexagonal) structure.

The thickness of the III-nitride material device region (e.g., which maycomprise at least one gallium nitride material layer) and the number ofdifferent layers within the III-nitride material device region aredictated, at least in part, on the application in which the structure isused. According to certain embodiments, at a minimum, the totalthickness of the III-nitride material device region (or any individuallayer within the III-nitride material device region) is sufficient topermit formation of the desired structure or device. The total thicknessof the III-nitride material device region is, according to someembodiments, greater than about 0.1 micron, though not always. In someembodiments, the total thickness is greater than about 2.0 microns, oreven greater than about 5.0 microns. In some embodiments, the thicknessof at least one layer within the III-nitride material device region isgreater than about 0.1 micron, greater than about 2.0 microns, orgreater than about 5.0 microns.

The optional III-nitride material nucleation layer, the optionalIII-nitride material transition layer, and the optional III-nitridematerial buffer layer are not typically (though may be) part of theactive region of devices formed from structures of the embodimentsdescribed herein. As described above, these layers may be formed tofacilitate deposition of the layer(s) of the III-nitride material deviceregion. However, in some cases, the optional III-nitride materialnucleation layer, the optional III-nitride material transition layer,and/or the optional III-nitride material buffer layer may have otherfunctions including functioning as a heat spreading layer that helpsremove heat from active regions of the semiconductor structure duringoperation of a device. For example, such transition layers that functionas heat spreading layers have been described in U.S. Patent ApplicationPublication No. 2002/0117695, published Aug. 29, 2002, entitled “GalliumNitride Materials Including Thermally-Conductive Regions,” which isincorporated herein by reference in its entirety for all purposes.

Active regions of devices formed from certain of the structuresdescribed herein may be formed, in part, in one or more layers of theIII-nitride material device region (e.g., gallium nitride materiallayers). Suitable gallium nitride material layer arrangements have beendescribed, for example, in U.S. Pat. No. 7,071,498, entitled “GalliumNitride Material Devices Including an Electrode-Defining Layer andMethods of Forming the Same,” issued on Jul. 4, 2006, which isincorporated herein by reference in its entirety for all purposes. Othercommonly used III-Nitride material device layers include channel layers,spacer layers, barrier layers, capping layers, and P-type layers usedunder the gate electrodes used for the design of enhancement mode(normally OFF) transistor designs. These III-Nitride material devicelayers may also include, according to certain embodiments, intentionallydoped layers in addition to various III-Nitride layers exhibitingdifferent alloy compositions.

The semiconductor structures described herein may, according to certainembodiments, form the basis of a variety of semiconductor devices.Suitable devices include, but are not limited to, transistors (e.g.,FETs), Schottky diodes, as well as light-emitting devices including LEDsand laser diodes. It may be particularly advantageous, according tocertain but not necessarily all embodiments, to use structures of theinvention in devices that operate at high frequencies. Non-limitingexamples of applications using III-nitride devices operating at higherfrequencies include power management discretes and integrated circuitsused to switch, rectify, monitor, or control electric power from asource to a load (e.g., buck converters, boost converters, half bridges,H-bridges, full bridges, three-phase bridges and multi-phase bridges).Other non-limiting examples of RF applications include discretes andintegrated circuits used for wireless and wireline communications, RFenergy, RF plasma lighting, wireless charging, RF induction andmicrowave heating, RF spark-plugs, ISM, medical devices, RADAR, andelectronic warfare and countermeasure devices. In certain embodiments,there may be integrated circuits and/or multiple dice on a chipcombining both RF devices and switching devices used to monitor, switch,or control the electric power delivery from a source to a load.

According to certain embodiments, the devices have active regions thatare typically, at least in part, formed within the III-nitride materialregion (e.g., in one or more layers of the III-nitride material deviceregion, such as one or more gallium nitride material layers). Accordingto some embodiments, the devices include a variety of other functionallayers and/or features (e.g., electrodes, dielectric layers, field platelayers, etc.).

According to certain embodiments, the semiconductor structure comprisesa transistor (e.g., a field effect transistor (FET)). The transistor cancomprise, according to certain embodiments, a source electrode and adrain electrode. The source electrode and the drain electrode can beelectronically isolated from each other. For example, in someembodiments, the source electrode and the drain electrode are spatiallyseparated by a dielectric material. In some embodiments, the transistorfurther comprises a gate electrode. The gate electrode may be a Schottkygate or an insulated gate electrode. According to certain embodiments,during use, application of a voltage at the gate electrode can createand/or modify an electric field at least partially positioned betweenthe source electrode and the drain electrode, such that electrons aretransferred from the source electrode to the drain electrode. Suitabletransistors (e.g., FETs) that may be used in association with certain ofthe embodiments described herein include depletion mode (normally-ON)transistors and enhancement mode (normally OFF) transistors. Atransistor can be associated with any of the semiconductor structuresdescribed elsewhere herein, including but not limited to those describedwith respect to FIGS. 1A-1B.

FIG. 2A is an exemplary cross-sectional schematic illustration of asemiconductor structure 200A comprising transistor 210, according tocertain embodiments. In FIG. 2A, transistor 210 comprises sourceelectrode 220 and drain electrode 230. Transistor 210 also comprisesgate electrode 240. Source electrode 220, drain electrode 230, and gateelectrode 240 are positioned on III-nitride material region 120. Thedevice also includes dielectric layer(s) 250. The dielectric layer canbe a passivating layer that protects and passivates the surface of theIII-nitride material region. Via 260 is formed within the dielectriclayer 250 in which gate electrode 240 is, in part, formed. In FIG. 2A,as described above, III-nitride material region 120 is formed directlyon substrate 110. It is also noted that the configuration of gateelectrode 240 in direct contact with the III-nitride material deviceregion 190 forms a Schottky-gated FET. In other embodiments (not shown)there may be an insulator layer formed between gate electrode 240 andIII-nitride material device region 190 configured as a Metal InsulatedField Effect Transistor (MISFET) or more generically an insulated gatetransistor. It should also be noted that, although FIG. 2A shows via 260formed within dielectric layer 250, in certain other embodiments, via260 could extend down and terminate within III-nitride material region120.

FIG. 2B is another exemplary cross-sectional schematic illustration of asemiconductor structure 200B comprising transistor 210 (e.g., a FET). InFIG. 2B, the electrodes of transistor 210 are formed over semiconductorstructure 100B, as illustrated in FIG. 1B. Unlike in FIG. 2A, theIII-nitride material region 120 is formed above optional diffusionbarrier region 140, and optional diffusion barrier region 140 is formedabove substrate 110.

The transistor structure illustrated in FIGS. 2A-2B is exemplary, andother structures could also be used. For example, FIGS. 2C-2E areschematic illustrations of another exemplary semiconductor structure200C, according to certain embodiments. FIG. 2C is a top view schematicillustration of semiconductor structure 200C. FIG. 2D is a front viewcross-sectional illustration of semiconductor structure 200C from FIG.2C, with the cross-section taken along line 2D shown in FIG. 2C. FIG. 2Eis a side view cross-sectional illustration of semiconductor structure200C from FIG. 2C, with the cross-section taken along line 2E shown inFIG. 2C.

In some embodiments, the transistor comprises one or more source padscoupled to a plurality of source finger electrode portions. For example,in FIG. 2C, transistor 210 of semiconductor structure 200C comprises aplurality of source pads 220A coupled to source finger electrodeportions 220B. In some embodiments, the transistor comprises one or moredrain pads coupled to a plurality of drain finger electrode portions.For example, in FIG. 2C, transistor 210 of semiconductor structure 200Ccomprises a plurality of drain pads 230A coupled to drain fingerelectrode portions 230B. According to certain embodiments, thetransistor comprises one or more gate pads coupled to a plurality ofgate finger electrode portions. For example, in FIG. 2C, transistor 210of semiconductor structure 200C comprises a plurality of gate pads 240Acoupled to gate finger electrode portions 240B via gate conductors 240C.Transistor 210 can also comprise other components, such as one or moredielectric layers, but such additional components are omitted from FIG.2C for purposes of clarity.

According to certain embodiments, the plurality of source fingerelectrode portions can be elongated (e.g., having an aspect ratio of atleast about 2:1, at least about 3:1, at least about 5:1, at least about10:1, or longer). In some embodiments, the plurality of source fingerelectrode portions are substantially parallel to each other (i.e., thelongitudinal axes of the source finger electrode portions are parallelto within about 5°, within about 2°, or within about 1° of each other).For example, in FIG. 2C, transistor 210 of semiconductor structure 200Ccomprises elongated source finger electrode portions 220B, which havelongitudinal axes (extending horizontally across the page) that areparallel to each other. In some embodiments, the plurality of drainfinger electrode portions can be elongated (e.g., having an aspect ratioof at least about 2:1, at least about 3:1, at least about 5:1, at leastabout 10:1, or longer). In some embodiments, the plurality of drainfinger electrode portions are substantially parallel to each other(i.e., the longitudinal axes of the drain finger electrode portions areparallel to within about 5°, within about 2°, or within about 1° of eachother). For example, in FIG. 2C, transistor 210 of semiconductorstructure 200C comprises elongated drain finger electrode portions 230B,which have longitudinal axes (extending horizontally across the page)that are parallel to each other. In certain embodiments, the pluralityof gate finger electrode portions can be elongated (e.g., having anaspect ratio of at least about 2:1, at least about 3:1, at least about5:1, at least about 10:1, or longer). In some embodiments, the pluralityof gate finger electrode portions are substantially parallel to eachother (i.e., the longitudinal axes of the gate finger electrode portionsare parallel to within about 5°, within about 2°, or within about 1° ofeach other). For example, in FIG. 2C, transistor 210 of semiconductorstructure 200C comprises elongated gate finger electrode portions 240B,which have longitudinal axes (extending horizontally across the page)that are parallel to each other.

According to certain embodiments, at least some of the source fingerelectrode portions are interdigitated with drain finger electrodeportions. In some embodiments, at least some of the drain fingerelectrode portions are interdigitated with source finger electrodeportions. Generally, a first finger electrode portion is said to be“interdigitated” with other finger electrode portions when thelongitudinal axis of the first finger electrode portion extends into aspatial cavity created by the other finger electrode portions. In FIG.2C, for example, drain finger electrode portion 230B-1 is interdigitatedwith source finger electrode portions 220B-1 and 220B-2.

It should be understood that other structures and devices may be withinthe scope of the present invention including structures and devices thatare not specifically described herein. Other structures may includeother layers and/or features, amongst other differences.

In some cases, a parasitic channel can be present at (or proximate to) asurface of the substrate (e.g., a top surface of the substrate). In someembodiments, the parasitic channel can be formed in a surface region ofthe substrate, such as the top surface region of the substrate. As notedelsewhere, the surface region of the substrate can include an externalsurface of a substrate and/or a portion of the substrate underneath andclose to the external surface. In some embodiments, the surface regionof the substrate extends to a depth of about 5 microns, to a depth ofabout 2 microns, to a depth of about 1 micron, to a depth of about 500nm, to a depth of about 200 nm, or less. For example, referring back toFIG. 1A, in some cases, a parasitic channel may be formed in surfaceregion 130 of substrate 110, at or near substrate surface 135. Theparasitic channel, when present, can generally be formed in a topsurface region of the substrate, and can extend for a depth (d) into thesubstrate surface. In most cases, the parasitic channel extends to thetop surface, but it is possible for the top surface region to include athin portion at the top surface that is not parasitic and for theparasitic channel to be beneath that portion and, thus, proximate to thetop surface. In general, depth (d) is the distance from the top surfaceat which the free carrier concentration of the parasitic channel isequal to the free carrier concentration of the bulk substrate. Thus,depth (d) depends, in part, on the free carrier concentration of thebulk substrate. In some embodiments, such as when the silicon substrateis highly resistive, the free carrier concentration of the bulksubstrate may be less than about 10¹⁴/cm³ (e.g., 10¹²/cm³). Typicalparasitic channel depths may be, in some cases, at least about 1 micronand may be less than about 5 microns. In some embodiments, the depth ofthe parasitic channel is less than or equal to about 5 microns, lessthan or equal to about 2 microns, less than or equal to about 1 micron,less than or equal to about 500 nm, less than or equal to about 200 nm,or less.

In some embodiments, the devices that are formed are free of parasiticchannels or include only low-conductivity parasitic channels. In certainembodiments, the surface region of a device (e.g., the top surfaceregion of the device) is free of a parasitic channel or includes only alow-conductivity parasitic channel. “Low-conductivity parasiticchannels” are parasitic channels having a peak free carrierconcentration that is less than about 10¹⁷/cm³ and/or having a totalintegrated surface region charge of less than about 10¹²/cm². In someembodiments, the low-conductivity parasitic channel has a peak freecarrier concentration that is less than about 10¹⁷/cm³, less than about5×10¹⁶/cm³, less than about 10¹⁶/cm³, or less than about 10¹⁵/cm³.Low-conductivity parasitic channels can have, in some embodiments, peakfree carrier concentrations as low as about 10¹⁴/cm³, or lower. In someembodiments, the low-conductivity parasitic channel has a totalintegrated surface region charge that is less than about 10¹¹/cm², lessthan about 10¹⁰/cm², less than about 10⁹/cm², or less than about10⁸/cm². Low-conductivity parasitic channels can have, in someembodiments, total integrated surface region charges as low as about10⁸/cm², as low as about 10⁵/cm², or lower. Portions of parasiticchannels having relatively low free carrier concentrations (e.g., lessthan about 10¹⁴/cm³) and/or relatively low total integrated surfaceregion charges may not have a significant, if any, impact on deviceperformance. “High-conductivity parasitic channels” are parasiticchannels having a peak free carrier concentration that is at least about10¹⁷/cm³ and having a total integrated surface region charge of at leastabout 10¹²/cm². In some cases, a high-conductivity parasitic channel canhave a peak free carrier concentration that is at least about5×10¹⁷/cm³, at least about 10¹⁸/cm³, or at least about 10¹⁹/cm³. In somecases, a high-conductivity parasitic channel can have a total integratedsurface region charge that is at least about 10¹³/cm², at least about10¹⁴/cm², at least about 10¹⁵/cm², or at least about 10¹⁶/cm².

In some embodiments, various of the processing (e.g., implantation)methods described herein can be used to reduce the peak free carrierconcentration from above a first threshold level to below a secondthreshold level (which can be the same as or different from the firstthreshold level). For example, in some embodiments, processing (e.g.,implantation) can be performed such that the peak free carrierconcentration is reduced from a level above about 10¹⁷/cm³ to belowabout 10¹⁷/cm³ (or below about 5×10¹⁶/cm³, below about 10¹⁶/cm³, belowabout 10¹⁵/cm³, or below about 10¹⁴/cm³); from a level above about5×10¹⁶/cm³ to below about 5×10¹⁶/cm³ (or below about 10¹⁶/cm³, belowabout 10¹⁵/cm³, or below about 10¹⁴/cm³); from a level above about10¹⁶/cm³ to a level below about 10¹⁶/cm³ (or below about 10¹⁵/cm³, orbelow about 10¹⁴/cm³); from a level above about 10¹⁵/cm³ to a levelbelow about 10¹⁵/cm³ (or below about 10¹⁴/cm³ or from a level aboveabout 10¹⁴/cm³ to a level below about 10¹⁴/cm³. Such reductions may beachieved within any of the regions described herein in which thereduction of free carrier concentrations is described.

In some embodiments, various of the processing (e.g., implantation)methods described herein can be used to reduce the total integratedsurface region charge from above a first threshold level to below asecond threshold level (which can be the same as or different from thefirst threshold level). For example, in some embodiments, processing(e.g., implantation) can be performed such that the total integratedsurface region charge is reduced from a level above about 10¹¹/cm² tobelow about 10¹¹/cm² (or below about 10¹⁰/cm²; below about 10⁹/cm²,below about 10⁸/cm², or below about 10⁵/cm²); from a level above about10¹⁰/cm² to below about 10¹⁰/cm² (or below about 10⁹/cm², below about10⁸/cm², or below about 10⁵ cm²); from a level above about 10⁹/cm² to alevel below about 10⁹/cm² (or below about 10⁸/cm², or below about10⁵/cm²); or from a level above about 10⁸/cm² to a level below about10⁸/cm² (or below about 10⁵/cm²). Such reductions may be achieved withinany of the regions described herein in which the reduction of freecarrier concentrations is described.

The peak free carrier concentration of a parasitic channel may depend ona number of factors including the particular method used to reduce freecarrier concentration. The peak free carrier concentration of aparasitic channel refers to the highest (or “peak”) concentration offree carriers within the parasitic channel, and may be measured usingstandard techniques known to those of ordinary skill in the artincluding spreading resistance profiling (SRP) and Secondary Ion MassSpectroscopy (SIMS). The free carrier concentration in the parasiticchannel typically decreases with distance from the substrate top surfaceor from a maximum free carrier concentration that is displaced somedepth below the substrate top surface. As one example, referring to FIG.1A, the free carrier concentration may, in some embodiments, decreasefrom top surface 135 downward through the thickness of substrate 110.The free carrier concentration in the parasitic channel can, accordingto certain embodiments, exhibit a profile resulting from SRP measurementtechniques similar to that shown in FIG. 5. In some embodiments, thefree carrier concentration exhibits a maximum, which is referred to asthe “peak free carrier concentration.” The peak free carrierconcentration can, in some cases, be located at or near a top surface ofthe substrate. For example, in FIG. 5, free carrier concentration curve510 exhibits a maximum substantially at or near the top surface of thesubstrate (e.g., surface 135 in the figures) indicated as point 512 inFIG. 5. In some cases, the free carrier concentration decreases to aminimum at some depth within the substrate (e.g., at a depth of about 1micron or less, at a depth of about 0.5 microns or less, or at a depthof about 0.2 microns or less). For example, in FIG. 5, free carrierconcentration curve 510 exhibits a minimum at point 514, whichcorresponds to a depth of about 1.33 microns. In some cases, thesubstrate background free carrier concentration dominates the profile atthe minimum.

As noted above, certain of the devices described herein can haverelatively low total integrated surface region charges. The “totalintegrated surface region charge” refers to the average amount of freecarriers per unit volume, determined by integrating the free carrierconcentration profile curve from a surface of the substrate to the depthat which the parasitic channel free carrier concentration matches thebackground free carrier concentration of the substrate. The free carrierconcentration profile curve can be determined using standard techniquesknown to those of ordinary skill in the art including spreadingresistance profiling (SRP) and Secondary Ion Mass Spectroscopy (SIMS).As one example, the total integrated surface region charge for thedevice shown in FIG. 5 would be calculated by determining the area undercurve 510 from x=0 (i.e., the surface of the substrate) to x=1.33 (i.e.,the location on the x-axis corresponding to the depth within thesubstrate at which the value of curve 510 equals the background freecarrier concentration, which is indicated by dotted line 516), anddividing the determined area by 1.33. In other words, the totalintegrated surface region charge for a given free carrier concentrationprofile is determined by integrating the free carrier concentrationprofile curve from x=0 (i.e., the surface of the device) to x=d (i.e.,the depth at which the parasitic channel free carrier concentrationmatches the background free carrier concentration of the substrate).

In some cases, the free carrier within the parasitic channel region maybe of a first type and the native background free carrier may be of theopposite type. For example, in some cases, the free carrier within theparasitic layer may be an acceptor, whereas the native background freecarrier concentration of the substrate may be a donor. As such, a P-Njunction can be formed in the substrate. For example, the P-N junctioncan comprise a P-type parasitic layer and an N-type background freecarrier region (e.g., of a high resistivity silicon substrate).According to certain embodiments, the P-N junction does not exist withinthe substrate prior to forming the III-nitride material region. Rather,the P-N junction and parasitic layer in the silicon substrate aregenerally formed in such embodiments during the semiconductor growthprocess of the III-nitride material region (which may include, forexample, the optional nucleation layer, the optional transitionlayer(s), the optional buffer layer(s), and/or optional layer(s) of theIII-nitride material device region).

One feature of certain embodiments of the invention is that the peakfree carrier concentration may be relatively low. For example, in someembodiments, the peak free carrier concentration may be less than about10¹⁷/cm³; in some cases, less than about 10¹⁶/cm³; in some cases, lessthan about 10¹⁵/cm³; in some cases, less than about 10¹⁴/cm³; in somecases, less than about 10¹³/cm³; or lower. In some embodiments, the peakfree carrier concentration is less than about 100 times the free carrierconcentration in the bulk of the substrate, less than about 10 times thefree carrier concentration in the bulk of the substrate, less than about5 times the free carrier concentration in the bulk of the substrate,less than about 2 times the free carrier concentration in the bulk ofthe substrate, less than about 1.5 times the free carrier concentrationin the bulk of the substrate, or less than about 1.1 times the freecarrier concentration in the bulk of the substrate. According to certainembodiments, both the peak free carrier concentration and the backgroundbulk substrate free carrier concentration are less than about 10¹³/cm³.Depth profiles may be generated using the SRP and SIMS techniques notedabove.

According to certain embodiments, the substrate includes a bulk regionbelow the top surface region. For example, referring to FIGS. 1A-1B,2A-2B, 2D-2E, 3A-3B, 3D-3E, 3G-3H, 3J, and 4A-4B, substrate 110 cancomprise bulk region 195 below top surface region 130. In someembodiments, the bulk region of the substrate has a lower peak freecarrier concentration than the top surface region. For example, the topsurface region of the substrate may have a relatively high peak freecarrier concentration (e.g., when a low-conductivity or ahigh-conductivity parasitic channel is present), and the bulk region ofthe substrate may have a lower peak free carrier concentration. In someembodiments, the bulk region of the substrate is doped with a first freecarrier type and the top surface region is doped with a second freecarrier type (e.g., a Group III species such as Al, Ga, and/or In). Insome embodiments, the second free carrier type is Al and/or Ga.According to certain embodiments, the peak free carrier concentration inthe bulk region is less than about 10¹³/cm³, less than about 10¹²/cm³,less than about 10¹¹/cm³, less than about 10¹⁰/cm³, or less than about10⁹/cm³.

As noted above, in some embodiments, semiconductor structures can beformed such that the adverse impact of an existing parasitic channel isreduced or eliminated. The impact of an existing parasitic channel canbe reduced or eliminated, for example, by implanting one or more speciesinto the semiconductor structure. Implanting the species into thesemiconductor structure can, according to certain embodiments, disruptthe parasitic channel and/or reduce the electronic conductivity withinthe parasitic channel, or eliminate the parasitic channel entirely.

In certain embodiments, a method of forming a semiconductor structure isdescribed comprising implanting a species having a relative atomic massof less than 5 into a substrate comprising silicon. Those of ordinaryskill in the art are familiar with “relative atomic mass” (alsosometimes referred to as “atomic weight”), which refers to the ratio ofthe average mass of atoms of an element to 1/12 of the mass of an atomof carbon-12 (known as the unified atomic mass unit). As one example,atomic hydrogen has a relative atomic mass of 1.008. Atomic helium has arelative atomic mass of 4.003. Examples of species having a relativeatomic mass of less than 5 include, but are not limited to, hydrogen andhelium. The species having a relative atomic mass of less than 5 can beatomic (e.g., atomic helium (He)), ionic (e.g., hydrogen cations (H⁺)also referred to herein as protons), or molecular (e.g., molecularhydrogen (H₂)). In some embodiments, a single species having a relativeatomic mass of less than 5 is implanted into the substrate, while inother embodiments, multiple species having a relative atomic mass ofless than 5 are implanted into the substrate.

In some embodiments, implanting the species having a relative atomicmass of less than 5 produces a surface region comprising alow-conductivity parasitic channel, or a surface region that does notcomprise a parasitic channel. For example, referring to FIG. 1A, in someembodiments, a species having a relative atomic mass of less than 5 canbe implanted into substrate 110. In some such embodiments, implantationof the species into substrate 110 can result in the formation of alow-conductivity parasitic channel in surface region 130. In some suchembodiments, implantation of the species into substrate 110 can resultin surface region 130 being free of a parasitic channel.

According to certain embodiments, during the implanting step, at least aportion of the species having a relative atomic mass of less than 5 isimplanted through a III-nitride material region of the semiconductorstructure. For example, referring to FIG. 1A, in some embodiments, aspecies having a relative atomic mass of less than 5 is implantedthrough III-nitride material region 120, through surface 135, and intosubstrate 110. In some such embodiments, the III-nitride material regionis formed over the substrate in a first step (e.g., via any of themethods described elsewhere herein), after which the species having arelative atomic mass of less than 5 is implanted through the III-nitridematerial region.

It should be understood that, as described herein, a species isconsidered to be implanted through a III-nitride material region when itis implanted through the finally formed III-nitride material region aswell as when it is implanted through only an intermediate portion of theIII-nitride material region, which may have only a fraction of thethickness of the fully-formed III-nitride material region. For example,if a species is implanted through a partially-formed III-nitridematerial region (or a partially-formed III-nitride material layer), andsubsequent to the implantation of the species, additional potion(s) ofIII-nitride material (or additional portion(s) of the layer ofIII-nitride material) are formed directly on the III-nitride materialthrough which the species was implanted, the species would still beconsidered to have been implanted through a III-nitride material region.

In certain embodiments, the species having a relative atomic mass ofless than 5 can be implanted through an optional III-nitride materialnucleation layer. For example, referring to FIG. 1D, in someembodiments, a species having a relative atomic mass of less than 5 canbe implanted through optional III-nitride material nucleation layer 155(and subsequently through surface 135 and into substrate 110). In somesuch embodiments, the optional III-nitride material nucleation layer (ora portion thereof) is formed over the substrate in a first step (e.g.,via any of the methods described elsewhere herein), after which thespecies having a relative atomic mass of less than 5 is implantedthrough the III-nitride material nucleation layer.

In some embodiments, the species having a relative atomic mass of lessthan 5 can be implanted through an optional III-nitride materialtransition layer. For example, referring to FIG. 1D, in someembodiments, a species having a relative atomic mass of less than 5 canbe implanted through optional III-nitride material transition layer 170(and subsequently through surface 135 and into substrate 110). In somesuch embodiments, the optional III-nitride material transition layer (ora portion thereof) is formed over the substrate in a first step (e.g.,via any of the methods described elsewhere herein), after which thespecies having a relative atomic mass of less than 5 is implantedthrough the III-nitride material transition layer.

In some embodiments, the species having a relative atomic mass of lessthan 5 can be implanted through an optional III-nitride material bufferlayer. For example, referring to FIG. 1D, in some embodiments, a specieshaving a relative atomic mass of less than 5 can be implanted throughoptional III-nitride material buffer layer 180 (and subsequently throughsurface 135 and into substrate 110). In some such embodiments, theoptional III-nitride material buffer layer (or a portion thereof) isformed over the substrate in a first step (e.g., via any of the methodsdescribed elsewhere herein), after which the species having a relativeatomic mass of less than 5 is implanted through the III-nitride materialbuffer layer.

In some embodiments, the species having a relative atomic mass of lessthan 5 can be implanted through at least one layer of a III-nitridematerial device region (e.g., a GaN device layer). For example,referring to FIG. 1D, in some embodiments, a species having a relativeatomic mass of less than 5 can be implanted through at least one layerof III-nitride material device region 190 (and subsequently throughsurface 135 and into substrate 110). In some such embodiments, a layerof the III-nitride material device region (or a portion thereof) isformed over the substrate in a first step (e.g., via any of the methodsdescribed elsewhere herein), after which the species having a relativeatomic mass of less than 5 is implanted through the layer of theIII-nitride material device region. In some such embodiments, the layerof the III-nitride material device region through which the species isimplanted can be an epitaxial III-nitride material device material layer(e.g., an epitaxial GaN layer). In some such embodiments, the layer ofthe III-nitride material device region through which the species isimplanted can be a monocrystalline III-nitride material device layer(e.g., a monocrystalline GaN layer).

According to certain embodiments, the species having a relative atomicmass of less than 5 can be implanted through a transistor (e.g., FET) ofthe semiconductor structure. For example, referring to FIG. 2A, in someembodiments a species having a relative atomic mass of less than 5 canbe implanted through transistor 210 and III-nitride material region 120(and subsequently through surface 135 and into substrate 110).

The species having a relative atomic mass of less than 5 can beimplanted, in some embodiments, between III-nitride material formationsteps. For example, in some embodiments, after the species has beenimplanted through a first III-nitride material region, a secondIII-nitride material region is formed over the first III-nitridematerial region through which the species was implanted. In some suchembodiments, the second III-nitride material can be formed directly onthe first III-nitride material region. In some such embodiments, thesecond III-nitride material can be a monocrystalline III-nitridematerial (e.g., a monocrystalline GaN layer). In some such embodiments,the second III-nitride material can be an epitaxial III-nitride material(e.g., an epitaxial GaN layer). In certain embodiments, the secondIII-nitride material region can include a 2DEG region. According tocertain embodiments, by forming the second III-nitride material afterthe implantation step has been performed, one can avoid damaging thesecond III-nitride material region via the implantation step. Forexample, according to some embodiments, a species (e.g., having arelative atomic mass of less than 5) can be implanted through anoptional III-nitride material nucleation layer, an optional III-nitridematerial transition layer, and/or an optional III-nitride materialbuffer layer. In some such embodiments, after the implantation step, atleast a layer of a III-nitride material device region can be formed overthe substrate (and/or the optional III-nitride material nucleationlayer, the optional III-nitride material transition layer, and/or theoptional III-nitride material buffer layer). In some embodiments, aspecies (e.g., having a relative atomic mass of less than 5) can beimplanted through an optional III-nitride material nucleation layerand/or an optional III-nitride material transition layer. In some suchembodiments, after the implantation step, a III-nitride material bufferlayer and/or at least a layer of a III-nitride material device regioncan be formed over the substrate (and/or the optional III-nitridematerial nucleation layer and/or the optional III-nitride materialtransition layer). In some embodiments, a species (e.g., having arelative atomic mass of less than 5) can be implanted through anoptional III-nitride material nucleation layer. In some suchembodiments, after the implantation step, a III-nitride materialtransition layer, a III-nitride material buffer layer, and/or at least alayer of a III-nitride material device region can be formed over thesubstrate (and/or the optional III-nitride material nucleation layer).According to certain embodiments, a species (e.g., having a relativeatomic mass of less than 5) can be implanted through an optionalIII-nitride material nucleation layer. In some such embodiments, afterthe implantation step, a III-nitride material transition layer, aIII-nitride material buffer layer, and/or at least a layer of aIII-nitride material device region can be formed over the substrate(and/or the optional III-nitride material nucleation layer).

According to certain embodiments, after the implantation has beenperformed, a transistor (e.g., a FET) may be formed over the III-nitridematerial region(s). For example, in some embodiments, a III-nitridematerial region is formed over the substrate, a species having arelative atomic mass of less than 5 is implanted into the substrate,after which one or more electrodes of the transistor are formed. Incertain embodiments, at least part of the transistor is formed prior tothe implantation of the species having a relative atomic mass of lessthan 5. For example, in some embodiments, a III-nitride material regionis formed over the substrate, a transistor is formed over theIII-nitride material region, and subsequently, a species having arelative atomic mass of less than 5 is implanted into the substrate(e.g., through the transistor and the III-nitride material region).

In some embodiments, a species having a relative atomic mass of lessthan 5 is implanted into the semiconductor substrate from the back sideof the substrate. In some such embodiments, transport of the implantedspecies through the III-nitride material region is eliminated, which, insome cases, can help preserve the functionality and/or structure of theIII-nitride material region.

Accordingly, certain inventive methods comprise implanting a specieshaving a relative atomic mass of less than 5 into a structure comprisinga III-nitride material region and a substrate comprising silicon,wherein at least a portion of the species is implanted through thesubstrate without being implanted through the III-nitride materialregion. This may be achieved, according to certain embodiments, byimplanting the species having a relative atomic mass of less than 5 intothe substrate via the back side of the substrate (i.e., the side of thesubstrate opposite the substrate surface over which the diffusionbarrier region, the III-nitride material region, and/or the transistoris positioned). One example of such an implantation method isillustrated in FIG. 4A. In FIG. 4A, species 315 is implanted throughback side 410 of substrate 110. Species 315 is not implanted, however,through III-nitride material region 120. In addition, species 315 is notimplanted through transistor 210 (e.g., when transistor 210 is presentduring the implantation step). Another example of such implantation isillustrated in FIG. 4B. In FIG. 4B, species 315 is implanted throughback side 410 of substrate 110. In the embodiment shown in FIG. 4B,however, species 315 is not implanted through III-nitride materialregion 120. In certain embodiments, the species having a relative atomicmass of less than 5 is not implanted through the optional III-nitridematerial nucleation layer. For example, in FIG. 4B, species 315 is notimplanted through optional III-nitride material nucleation layer 155. Incertain embodiments, the species having a relative atomic mass of lessthan 5 is not implanted through the optional III-nitride materialtransition layer. For example, in FIG. 4B, species 315 is not implantedthrough optional III-nitride material transition layer 170. In certainembodiments, the species having a relative atomic mass of less than 5 isnot implanted through the III-nitride material device region. Forexample, in FIG. 4B, species 315 is not implanted through III-nitridematerial device region 190.

In certain embodiments, the portion of the species that is not implantedthrough a particular region (e.g., the III-nitride region) may beimplanted into the region but not completely through the region. Forexample, referring to FIGS. 4A-4B, in some cases, at least a portion ofspecies 315 is implanted through substrate 110, across surface 135, andinto III-nitride material region 120, but is not implanted throughIII-nitride material region 120. In other cases, the portion of thespecies that is not implanted through a particular region is not evenimplanted into the region. For example, referring to FIGS. 4A-4B, insome embodiments, at least a portion of species 315 is implanted intosubstrate 110 without crossing surface 135 (and, thus, without passinginto III-nitride material region 120). Thus, said species is not evenimplanted into III-nitride material region 120 (and said species wouldnot be implanted through III-nitride material region 120).

According to certain embodiments, at least a portion of the specieshaving a relative atomic mass of less than 5 is not implanted throughthe transistor. For example, in FIG. 4B, species 315 is not implantedthrough transistor 210 (e.g., when transistor 210 is present during theimplantation step). In certain embodiments, the transistor may not beformed until after the implantation of the species having a relativeatomic mass of less than 5 is implanted.

Various of the implantation methods described herein can producesemiconductor structures in which the substrate comprises a regionhaving a relatively high concentration of species having a relativeatomic mass of less than 5. In some embodiments, at least a region ofthe substrate comprises at least one species having a relative atomicmass of less than 5 at a concentration of at least about 10¹⁹/cm³ (or atleast about 10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³,at least about 10²³/cm³, or more). The concentration of species havingrelative atomic masses of less than 5 can be measured using standardtechniques known to those of ordinary skill in the art includingspreading resistance profiling (SRP) and Secondary Ion Mass Spectroscopy(SIMS). In embodiments in which hydrogen is used as an implantedspecies, hydrogen forward scattering spectrometry (HFS) can be used todetermine the concentration of hydrogen within regions of thesemiconductor structure.

In some such embodiments, the region having a relatively highconcentration of species having a relative atomic mass of less than 5can be part of a surface region and/or a parasitic channel in thesubstrate. For example, referring to FIGS. 3A-3B, in some embodiments,one or more of regions 330 can include species having a relative atomicmass of less than 5 at a concentration within any of the rangesdescribed above.

According to certain embodiments in which species having a relativeatomic mass of less than 5 are implanted into the substrate, thesubstrate may also include a relatively high peak concentration of GroupIII species (e.g., Al, Ga, In, Tl, and B). The relatively high peakconcentration(s) of Group III species can be present before theimplantation or species having a relative atomic mass of less than 5,according to some embodiments. In certain embodiments, the relativelyhigh peak concentration(s) of Group III species can be present after theimplantation of species having a relative atomic mass of less than 5.The presence of the species having a relative atomic mass of less than 5can reduce the electronic conductivity of the Group III species,inhibiting the formation of a high conductivity parasitic channel. Insome embodiments in which species having a relative atomic mass of lessthan 5 are implanted into the substrate, the peak of the sum of theconcentrations of Group III species in the substrate (e.g., the peak ofthe sum of the concentrations of Al, Ga, In, Tl, and B in the substrate)is at least about 10¹⁷/cm³, at least about 10¹⁸/cm³, at least about10¹⁹/cm³, at least about 10²⁰/cm³, at least about 10²¹/cm³, at leastabout 10²²/cm³, or at least about 10²³/cm³. In some embodiments in whichspecies having a relative atomic mass of less than 5 are implanted intothe substrate, the peak of the sum of the concentrations of Al, Ga, andIn in the substrate is at least about 10¹⁷/cm³, at least about 10¹⁸/cm³,at least about 10¹⁹/cm³, at least about 10²⁰/cm³, at least about10²¹/cm³, at least about 10²²/cm³, or at least about 10²³/cm³. In someembodiments in which species having a relative atomic mass of less than5 are implanted into the substrate, the peak concentration of Al, Ga,and/or In in the substrate is at least about 10¹⁷/cm³, at least about10¹⁸/cm³, at least about 10¹⁹/cm³, at least about 10²⁰/cm³, at leastabout 10²¹/cm³, at least about 10²²/cm³, or at least about 10²³/cm³.

In some embodiments, the substrate comprises a high-conductivityparasitic channel before the implantation of species having a relativeatomic mass of less than 5 and a low-conductivity parasitic channel (orno parasitic channel) after the implantation of species having arelative atomic mass of less than 5. In some embodiments, the bulkregion of the substrate has a lower peak free carrier concentration thanthe top surface region of the substrate before and/or after theimplantation of species having a relative atomic mass of less than 5. Insome embodiments, the bulk region of the substrate is doped with a firstfree carrier type and the top surface region is doped with a second freecarrier type (e.g., a Group III species such as Al, Ga, In, and/or Tl)before and/or after the implantation of species having a relative atomicmass of less than 5. According to certain embodiments, the peak freecarrier concentration in the bulk region is less than about 10¹³/cm³,less than about 10¹²/cm³, less than about 10¹¹/cm³, less than about10¹⁰/cm³, or less than about 10⁹/cm³ before and/or after theimplantation of species having a relative atomic mass of less than 5.

In some embodiments, at least one species is implanted into thesubstrate in a spatially defined pattern. As a result, certain of theinventive semiconductor structures described herein include specieslocated within the substrate (e.g., in a surface region of a substrate,such as surface region 130 in the figures) in a spatially definedpattern. Implanting species in a spatially defined pattern can allow oneto select the regions of the semiconductor structure through which thespecies is transported, potentially limiting or eliminating damagecaused by the implanted species within certain regions. Patternedimplantation of the species can be performed using any of thesemiconductor structures described herein.

Those of ordinary skill in the art would be capable of determining theamount of implanted species within various regions of a givensemiconductor device using, for example, Secondary Ion Mass Spectroscopy(SIMS).

Certain embodiments comprise implanting a species into the substrate(e.g., into a surface region of the substrate) such that the implantedspecies forms a pattern spatially defined across at least one lateraldimension of the substrate (and, in some embodiments, across two lateraldimensions of the substrate). A “lateral dimension” of a substratecorresponds to a dimension of the substrate that is perpendicular to thethickness of the substrate. The lateral dimensions of the substrate candefine the face of the substrate on which the barrier region, theIII-nitride material region, and/or transistors (or other devices) areformed. The thickness of the substrate can be the smallest of the threecoordinate dimensions of the substrate. For example, in the case of asilicon wafer, the wafer can have a thickness corresponding to itsthinnest coordinate dimension and two lateral dimensions—which form theface of the wafer—each perpendicular to each other, and eachperpendicular to the thickness of the wafer. Referring to FIG. 1A as anon-limiting example, substrate 110 comprises thickness 196 and lateraldimension 197. (Substrate 110 also has a second lateral dimension thatextends into and out of the plane across which FIG. 1A is drawn.) Incertain embodiments, the implanted species can form a pattern that isalso spatially defined across the second lateral dimension of thesubstrate. In some embodiments, the spatially defined pattern in whichthe species is implanted can vary across the depth of (i.e., through thethickness of) the substrate. Implanting species according to suchpatterns can provide, according to certain embodiments, a number ofadvantages discussed in more detail below.

In some embodiments, at least one species can be implanted into asurface region of the substrate (e.g., into surface region 130 in thefigures) such that the implanted species forms a pattern spatiallydefined across at least one lateral dimension (and, in some embodiments,across two lateral dimensions) of the surface region of the substrate.In some such embodiments, the species that is implanted into thesubstrate can be implanted through a III-nitride material region andsubsequently into the substrate. In some such embodiments, after theimplanting step, the implanted species is present within at least aportion of the surface region of the substrate at a concentration of atleast about 10¹⁹/cm³ (and/or, in some embodiments, at least about10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³, or at leastabout 10²³/cm³). According to some such embodiments, after theimplanting step, at least a second portion of the surface region of thesubstrate is substantially free of the first implanted species (i.e.,the implanted species is present within the second portion in an amountof less than about 10¹⁵/cm³, and, in some cases, less than about10¹⁴/cm³, less than about 10¹³/cm³, less than about 10¹²/cm³, less thanabout 10¹¹/cm³, less than about 10¹⁰/cm³, or lower).

According to certain embodiments, the species that is implanted into thesubstrate in a spatially defined pattern can have a relative atomic massof less than 5. The species having a relative atomic mass of less than 5can be atomic (e.g., atomic helium (He)), ionic (e.g., hydrogen cations(H⁺)), or molecular (e.g., molecular hydrogen (H₂)). Other species couldalso be used including, but not limited to, p-type dopants (e.g., boron,aluminum, gallium, and/or indium) and n-type dopants (e.g., nitrogen,phosphorus, oxygen, and/or arsenic). In some embodiments, the spatiallydefined pattern in which the species is implanted can be a preselectedspatially defined pattern. Such preselected spatially defined patterningmay be achieved, for example, using an implantation mask. According tocertain embodiments, the implanting is performed through a mask suchthat a first portion of the semiconductor structure is not substantiallyexposed to the implanted species and a second portion of thesemiconductor structure is exposed to the implanted species. Forexample, an implantation mask comprising one or more open regions (whichallow the implanted species to pass through the mask) and one or moreblocking regions (which inhibit or prevent the implanted species frompassing through the mask) can be employed. An exemplary schematicillustration showing such patterning is shown in FIG. 3A. In FIG. 3A,mask 300 comprises open regions 310, which allow implanted species 315Ato pass through the mask and blocking region 320 which inhibits (orprevents) implanted species 315B from passing through mask 300. Anotherexample of such patterning is shown in FIG. 3B, in which mask 300comprises open regions 310, which allow implanted species 315A to passthrough the mask and blocking region 320 which inhibits (or prevents)implanted species 315B from passing through mask 300. Although the openregions in FIGS. 3A and 3B are substantially rectangular in shape, maskopenings with other shapes (e.g., circular or substantially circular,hexagonal or substantially hexagonal, parallelograms, etc.) may also beused.

According to some embodiments, the patterned implantation is performedsuch that at least one region of a parasitic channel (e.g., within asurface region of the substrate, such as surface region 130 in thefigures) is exposed to the implanted species. In some such embodiments,at least a portion of the III-nitride material region is not exposed tothe implanted species. In certain such embodiments, damage of theportions of the III-nitride material region that are not exposed to theimplanted species can be avoided. For example, referring to FIGS. 3A-3B,in some embodiments, regions 330 of the parasitic channel within surfaceregion 130 (and/or regions 331 of the III-nitride material region 120)are exposed to the implanted species while region 335 of III-nitridematerial region 120 (which region 335 can include a 2DEG region) andregion 340 of a parasitic channel within surface region 130 are notexposed to the implanted species. According to some such embodiments,the structure of the material within region 335 (including, in someembodiments, the 2DEG region within region 335) is preserved while thenumber of free carriers within regions 330 is reduced or eliminated.According to certain embodiments, a 2DEG within region 335 is preservedwhile the 2DEG within regions 331 is reduced, eliminated, orinterrupted, and the number of free carriers within regions 330 isreduced or eliminated.

According to certain embodiments, patterned implantation can reduce theconductivity of the parasitic channel region (or eliminate the parasiticchannel region) while preserving the structure of one or more functionalregions of the III-nitride material region (e.g., a 2DEG region). Insome embodiments, after the species has been implanted into thesemiconductor structure, the species is not substantially present withinthe III-nitride material region. Thus, in some embodiments, theIII-nitride material region of the semiconductor structure (throughwhich the species may be implanted) can be substantially free of theimplanted species (i.e., the implanted species is not present or ispresent in an amount of less than about 10¹⁵/cm³, and, in some cases,less than about 10¹⁴/cm³, less than about 10¹³/cm³, less than about10¹²/cm³, less than about 10¹¹/cm³, less than about 10¹⁰/cm³, or lower).In some embodiments, the III-nitride material region comprises a 2DEGregion, and the 2DEG region (through which the species may be implanted)is substantially free of the first implanted species (e.g., after thespecies is implanted).

In some embodiments, the implanted species is not implanted into atleast a portion of a region between a source of the transistor and adrain of the transistor. The region between the source of the transistorand the drain of the transistor through which the species is notimplanted may correspond to, in some embodiments, a 2DEG region. Thus,in some embodiments, the III-nitride material region of thesemiconductor comprises a 2DEG region that is substantially free of theimplanted species (i.e., the implanted species is not present or ispresent in an amount of less than about 10¹⁵/cm³, and, in some cases,less than about 10¹⁴/cm³, less than about 10¹³/cm³, less than about10¹²/cm³, less than about 10¹¹/cm³, less than about 10¹⁰/cm³, or lower).According to certain embodiments, the species is not implanted into atleast a portion of a region between a source of the transistor and agate of the transistor. In some embodiments, the species is notimplanted into at least a portion of a region between a drain of thetransistor and a gate of the transistor.

In certain embodiments, after the implanting step, at least a portion ofthe substrate underneath a channel between a source of the transistorand a drain of the transistor is substantially free of the implantedspecies (i.e., the implanted species is not present or is present in anamount of less than about 10¹⁵/cm³, and, in some cases, less than about10¹⁴/cm³, less than about 10¹³/cm³, less than about 10¹²/cm³, less thanabout 10¹¹/cm³, less than about 10¹⁰/cm³, or lower). In someembodiments, at least a portion of the substrate underneath a regionbetween a source of the transistor and a gate of the transistor issubstantially free of the implanted species. For example, referring toFIGS. 3A-3B, in some embodiments, region 340 (which is underneath aregion between source 220 and drain 230 of transistor 210 (and which hasportions between source 220 and gate 240 of transistor 210, and betweengate 240 and drain 230 of transistor 210) is substantially free of theimplanted species.

In some embodiments, the implanted species is not implanted into atleast a portion of a region between at least a portion of a sourceelectrode of the transistor and at least a portion of a drain electrodeof the transistor. For example, in some embodiments, the implantedspecies is not implanted into at least a portion of a region between asource finger electrode portion and a drain finger electrode portion.Such arrangement of the implanted species can be achieved, for example,using an implantation mask. FIG. 3C is a top-view schematic illustrationshowing an embodiment in which a mask comprising blocking regions 320(shown as shaded areas inside dotted rectangles) is used to avoidimplanting a species in multiple regions between source finger electrodeportions and drain finger electrode portions of a transistor (which cancorrespond to transistor 210 of semiconductor structure 200C from FIGS.2C-2E). FIG. 3D is a front view cross-sectional illustration ofsemiconductor structure 200C from FIG. 3C, with the cross-section takenalong line 3D shown in FIG. 3C. FIG. 3E is a side view cross-sectionalillustration of semiconductor structure 200C from FIG. 3C, with thecross-section taken along line 3E shown in FIG. 3C. As shown in FIGS.3D-3E, mask 300 comprises open regions 310, which allow implantedspecies 315A to pass through the mask and blocking region 320 whichinhibits (or prevents) implanted species 315B from passing through mask300. As a result, in some embodiments, regions 330 of the parasiticchannel within surface region 130 (and/or regions 331 of the III-nitridematerial region 120) are exposed to the implanted species while region335 of III-nitride material region 120 (which region 335 can include a2DEG region) and region 340 of a parasitic channel within surface region130 are not exposed to the implanted species. According to some suchembodiments, the structure of the material within region 335 (including,in some embodiments, the 2DEG region within region 335) is preservedwhile the number of free carriers within regions 330 is reduced oreliminated. According to certain embodiments, a 2DEG within region 335is preserved while the 2DEG within regions 331 is reduced, eliminated,or interrupted and the number of free carriers within regions 330 isreduced or eliminated.

According to certain embodiments, the implanted species is implantedinto at least a portion of a substrate region (e.g., a substrate surfaceregion) underneath a source electrode, a gate electrode, and/or a drainelectrode of the transistor. In some embodiments, implantation of thespecies can be inhibited across at least a portion of (and, in someembodiments, a relatively large percentage of (e.g., at least about 50%of, at least about 75% of, or at least about 90% of)) the active area ofthe transistor, while that species is implanted into one or more regionsunderneath a source electrode, a drain electrode, and/or a gateelectrode of the transistor. For example, in some embodiments,implantation of the species can be inhibited within at least onesubstrate surface region between a source electrode and a gate electrodeand/or within at least one substrate surface region between a gateelectrode and a drain electrode while, at the same time, the species canbe implanted into at least one substrate surface region underneath asource electrode, a gate electrode, and/or a drain electrode. Suchpatterned implantation can be achieved, for example, using animplantation mask. For example, in some embodiments, the implantationmask can be used to inhibit the implantation of species within an activearea of the transistor (and, in some cases, across a relatively largepercentage of the active area of the transistor) but can allow forimplantation of species underneath one or more regions underneath thesource electrode (e.g., a source electrode pad), the gate electrode(e.g., a gate electrode pad), and/or the drain electrode (e.g., a drainelectrode pad).

FIG. 3F is a top-view schematic illustration showing an embodiment inwhich a mask comprising blocking region 320 (shown as a shaded areainside a dotted rectangle) is used to avoid implanting within a largearea of the active region of transistor 210 of semiconductor structure200C (which can correspond to transistor 210 of semiconductor structure200C from FIGS. 2C-2E). FIG. 3G is a front view cross-sectionalillustration of semiconductor structure 200C from FIG. 3F, with thecross-section taken along line 3G shown in FIG. 3F. FIG. 3H is a sideview cross-sectional illustration of semiconductor structure 200C fromFIG. 3F, with the cross-section taken along line 3H shown in FIG. 3F. Inthe exemplary embodiment shown in FIGS. 3F-3H, substantially the entireactive area of the transistor is masked to prevent implantation of thespecies into the active area. However, implantation of the implantedspecies can occur outside of the active region (e.g., in the fieldregion of the transistor, such as under the source, gate, and/or drainpads; in the isolation regions of the transistor, and/or in sawstreet(s) between the transistor and an adjacent transistor). Accordingto certain embodiments, by implanting outside the active area of thetransistor, a high-conductivity parasitic channel area can remain intactin the active area of the transistor, while only a low-conductivityparasitic channel (or no parasitic channel) remains outside the activearea, due to interaction of the implanted species into the surfaceregion of the substrate. For example, as shown in FIGS. 3F-3H, mask 300comprises open regions 310, which allow implanted species 315A to passthrough the mask and blocking region 320 which inhibits (or prevents)implanted species 315B from passing through mask 300. As a result, insome embodiments, regions 330 of the parasitic channel within surfaceregion 130 (and/or regions 331 of the III-nitride material region 120)are exposed to the implanted species while region 335 of III-nitridematerial region 120 (which region 335 can include a 2DEG region) andregion 340 of a parasitic channel within surface region 130 are notexposed to the implanted species. According to some such embodiments,the structure of the material within region 335 (including, in someembodiments, the 2DEG region within region 335) is preserved while thenumber of free carriers within regions 330 is reduced or eliminated.According to certain embodiments, a 2DEG within region 335 is preservedwhile the 2DEG within regions 331 is reduced, eliminated, or interruptedand the number of free carriers within regions 330 is reduced oreliminated.

As noted above, in some embodiments, a species can be implanted in afield region of a semiconductor structure to reduce the conductivity ofor eliminate a parasitic channel. Such implantation schemes may beuseful, for example, in eliminating high-conductivity parasitic channelsunderneath contact pads of a transistor and/or between adjacenttransistors of a semiconductor structure.

In some embodiments, the semiconductor structure comprises a firsttransistor and a second transistor laterally spaced apart from the firsttransistor. For example, referring to FIGS. 3I-3J, semiconductorstructure 390 comprises a first transistor 210-1 and a second transistor210-2 laterally spaced apart from first transistor 210-1 (i.e., spacedapart along lateral dimension 197 of substrate 110). In certainembodiments, more than two transistors may be present. For example, inFIGS. 3I-3J, semiconductor structure 390 also comprises an optionalthird transistor 210-3 laterally spaced apart from first transistor210-1 and second transistor 210-2, and an optional fourth transistor210-4 laterally spaced apart from first transistor 210-1, secondtransistor 210-2, and third transistor 210-3. In FIGS. 3I-3J, theelectrodes (e.g., the source, drain, and gate electrodes) and theinsulating materials between the electrodes are shown simply as block362, for purposes of maintaining clarity in the figures. Accordingly,each of blocks 362A, 362B, 362C, and 362D can include a sourceelectrode, a gate electrode, a drain electrode, and/or one or moredielectric layers, arranged in any suitable configuration (including,but not limited to, any of the configurations described herein).

According to certain embodiments, after an implanting step, at least aportion of the surface region of the substrate between the firsttransistor and the second transistor has a concentration of theimplanted species of at least about 10¹⁹/cm³ (or, in some embodiments,at least about 10²⁰/cm³, at least about 10²¹/cm³, at least about10²²/cm³, at least about 10²³/cm³, or more). For example, referring toFIG. 3J, in some embodiments, region 330 of surface region 130 ofsubstrate 110 between first transistor 210-1 and second transistor 210-2(and/or between second transistor 210-2 and third transistor 210-3,and/or between third transistor 210-3 and fourth transistor 210-4) has aconcentration of the implanted species of at least about 10¹⁹/cm³.According to certain embodiments, after an implanting step, the presenceof the implanted species within the surface region of the substratebetween the first transistor and the second transistor (and/or betweenthe second transistor and the third transistor, and/or between the thirdtransistor and the fourth transistor) can reduce the conductivity of aparasitic channel region within surface region 130 of substrate 110 suchthat a formerly high-conductivity parasitic channel region becomes alow-conductivity parasitic channel region or ceases to be a parasiticchannel region. According to certain embodiments, after an implantingstep, at least a portion of the surface region of the substrate belowthe first transistor and/or the second transistor is substantially freeof the implanted species (i.e., the implanted species is not present oris present in an amount of less than about 10¹⁵/cm³, and, in some cases,less than about 10¹⁴/cm³, less than about 10¹³/cm³, less than about10¹²/cm³, less than about 10¹¹/cm³, less than about 10¹⁰/cm³, or lower).For example, referring to FIG. 3J, in some embodiments, region 340 underfirst transistor 210-1 can be substantially free of the implantedspecies. In some embodiments, region 340 under second transistor 210-2(and/or third transistor 210-3, and/or fourth transistor 210-4) can besubstantially free of the implanted species.

According to certain embodiments, after an implanting step, the presenceof the implanted species within the surface region of the substratebetween the first transistor and the second transistor (and/or betweenthe second transistor and the third transistor, and/or between the thirdtransistor and the fourth transistor) can reduce the conductivity of aparasitic channel region within the surface region of the substrate suchthat a formerly high-conductivity parasitic channel region becomes alow-conductivity parasitic channel region (or ceases to be a parasiticchannel region), while a 2DEG within the III-nitride material region isreduced, eliminated, or interrupted.

FIGS. 3K-3M are top-view schematic illustrations showing exemplaryimplantation masks that can be used to implant one or more species intoa semiconductor structure comprising multiple devices (e.g.,transistors, such as FETs), according to certain embodiments.Transistors can be laterally spaced across the top surface of substrate,for example, in an array. For example, in FIGS. 3K-3M, transistors 210(including transistors 210-1, 210-2, 210-3, and 210-4) are arranged in atwo-dimensional array. The mask can include multiple blocking regions,which can inhibit or prevent the implantation of species into theunderlying semiconductor structure. For example, in FIGS. 3K-3M, theillustrated masks include blocking regions 320, which inhibit or preventimplantation of species into the semiconductor structure underlyingregions 320. FIG. 3K includes a pattern similar to the pattern describedwith respect to FIG. 3F, repeated over each of the transistors on thesubstrate. FIG. 3L includes a pattern similar to the pattern describedwith respect to FIG. 3C, repeated over each of the transistors on thesubstrate. In certain embodiments, the mask can be configured such openregions cover a relatively large percentage (e.g., at least about 80%,at least about 90%, at least about 95%, or at least about 99%) of thearea of the substrate covered by electrode pads, and blocking regionscover a relatively large percentage (e.g., at least about 80%, at leastabout 90%, at least about 95%, or at least about 99%) of the remainingsubstrate. One particular example is shown in FIG. 3M, in which the maskcomprises open regions 310 positioned over contact pads of transistors210, and blocking region 320 covers the remaining part of substrate 110.Although the open regions in FIGS. 3K-3M are substantially rectangularin shape, mask openings with other shapes (e.g., circular orsubstantially circular, hexagonal or substantially hexagonal,parallelograms, etc.) may also be used.

Certain embodiments comprise implanting a first species and a secondspecies into the semiconductor structure. According to certain suchembodiments, the first species and the second species can be implantedinto the semiconductor structure at different depths. This can produce,for example, a semiconductor structure in which a first species isimplanted at a first depth (e.g., with a surface region of thesubstrate) and a second species is present at a different depth (e.g.,within the III-nitride material region, such as within a 2DEG region).

According to certain embodiments, the first implanted species and thesecond implanted species are compositionally different. In someembodiments, the first species that is implanted into the semiconductorstructure can have a relative atomic mass of less than 5. The specieshaving a relative atomic mass of less than 5 can be atomic (e.g., atomichelium (He)), ionic (e.g., hydrogen cations (H⁺)), or molecular (e.g.,molecular hydrogen (H₂)). In certain embodiments, more than one firstspecies having an atomic mass of less than 5 may be implanted. In someembodiments, the second species that is implanted into the semiconductorstructure can be a p-type dopant (e.g., boron, aluminum, gallium, and/orindium) or an n-type dopants (e.g., nitrogen, oxygen, phosphorus, and/orarsenic).

In some embodiments, the first species is implanted into the substrateof the semiconductor structure, such as into a surface region of thesubstrate (e.g., surface region 130 in the figures). In some suchembodiments, the first implanted species is arranged within thesubstrate (e.g., within the surface region of the substrate) in a firstpattern spatially defined across at least one lateral dimension of thesubstrate (and, in some embodiments, across two lateral dimensions ofthe substrate). Various such patterns are described above, for example,with respect to FIGS. 3A-3J. In some embodiments, during the implantingof the first species, at least a portion of the first species isimplanted through the III-nitride material region. For example,referring to FIGS. 3A-3B, in some embodiments, a first species isimplanted through III-nitride material region 120 into surface region130 of substrate 110 (e.g., into regions 330 of surface region 130).Similarly, referring to FIGS. 3D-3E and 3G-3H, in some embodiments, afirst species is implanted through III-nitride material region 120 intosurface region 130 of substrate 110 (e.g., into regions 330 of surfaceregion 130). According to certain embodiments, after the first speciesis implanted, the first species is present within at least a portion ofthe surface region of the substrate at a concentration of at least about10¹⁹/cm³ (or, in some embodiments, at least about 10²⁰/cm³, at leastabout 10²¹/cm³, at least about 10²²/cm³, at least about 10²³/cm³, ormore). For example, referring to FIGS. 3A-3B, in some embodiments, aftera first species is implanted through III-nitride material region 120into surface region 130 of substrate 110 (e.g., into regions 330 ofsurface region 130), the concentration of the first species withinsurface region 130 (e.g., within region 330 of surface region 130) canbe at least about 10¹⁹/cm³. As another example, referring to FIGS. 3D-3Eand 3G-3H, in some embodiments, after a first species is implantedthrough III-nitride material region 120 into surface region 130 ofsubstrate 110 (e.g., into regions 330 of surface region 130), theconcentration of the first species within surface region 130 (e.g.,within region 330 of surface region 130) can be at least about 10¹⁹/cm³.

In some embodiments, after the first species is implanted, at least asecond portion of the surface region of the substrate is substantiallyfree of the first implanted species (i.e., the first implanted speciesis not present or is present in an amount of less than about 10¹⁵/cm³,and, in some cases, less than about 10¹⁴/cm³, less than about 10¹³/cm³,less than about 10¹²/cm³, less than about 10¹¹/cm³, less than about10¹⁰/cm³, or lower). For example, referring to FIGS. 3A-3B, in someembodiments, after a first species is implanted through III-nitridematerial region 120 into surface region 130 of substrate 110 (e.g., intoregions 330 of surface region 130), region 340 of surface region 130 ofsubstrate 110 can be substantially free of the first implanted species.As another example, referring to FIGS. 3D-3E and 3G-3H, according tocertain embodiments, after a first species is implanted throughIII-nitride material region 120 into surface region 130 of substrate 110(e.g., into regions 330 of surface region 130), region 340 of surfaceregion 130 of substrate 110 can be substantially free of the firstimplanted species.

According to certain embodiments, after the first species is implanted,the III-nitride material region is substantially free of the firstspecies (i.e., the first species is not present or is present in anamount of less than about 10¹⁵/cm³, and, in some cases, less than about10¹⁴/cm³, less than about 10¹³/cm³, less than about 10¹²/cm³, less thanabout 10¹¹/cm³, less than about 10¹⁰/cm³, or lower). For example,referring to FIGS. 3A-3B, in some embodiments, after a first species isimplanted through III-nitride material region 120 into surface region130 of substrate 110 (e.g., into regions 330 of surface region 130),III-nitride material region 120 can be substantially free of the firstimplanted species. As another example, referring to FIGS. 3D-3E and3G-3H, according to certain embodiments, after a first species isimplanted through III-nitride material region 120 into surface region130 of substrate 110 (e.g., into regions 330 of surface region 130),III-nitride material region 120 can be substantially free of the firstimplanted species. In some such embodiments, the III-nitride materialregion comprises a 2DEG region, and the 2DEG region (e.g., after thefirst species is implanted) is substantially free of the first implantedspecies. For example, referring to FIGS. 3A-3B, in some embodiments,III-nitride material region 120 comprises a 2DEG region and, after afirst species is implanted through III-nitride material region 120 intosurface region 130 of substrate 110 (e.g., into regions 330 of surfaceregion 130), the 2DEG region can be substantially free of the firstimplanted species. As another example, referring to FIGS. 3D-3E and3G-3H, according to certain embodiments, III-nitride material region 120comprises a 2DEG region and, after a first species is implanted throughIII-nitride material region 120 into surface region 130 of substrate 110(e.g., into regions 330 of surface region 130), the 2DEG region can besubstantially free of the first implanted species.

In certain embodiments, the second species is implanted into aIII-nitride material region of the semiconductor structure. The secondimplanted species may be arranged, according to certain embodiments,within the III-nitride material region in a second pattern spatiallydefined across at least one lateral dimension (and, in some embodiments,across two lateral dimensions) of the substrate. The second implantedspecies may also be arranged within the III-nitride material region suchthat the second species forms a pattern spatially defined across atleast one lateral dimension (and, in some embodiments, across twolateral dimensions) of the III-nitride material region.

According to some embodiments, the first implanted species can be usedto form a spatially defined implantation pattern in a parasitic channelregion within a semiconductor structure, and the second implantedspecies can be used to form a spatially defined implantation pattern ina III-nitride material region (e.g., a 2DEG region within a III-nitridematerial region) within the semiconductor structure. The first implantedspecies can be used, in some such embodiments, to reduce or eliminatethe conductivity of certain regions of a parasitic channel within asurface region of a substrate (e.g., as described above). The secondimplanted species can be used, in some such embodiments, to reduce oreliminate the conductivity of certain portions of a 2DEG region within aIII-nitride material region of the semiconductor structure. In someembodiments, the second implanted species can be used as an implantisolation step (e.g., in addition to or instead of an etched isolationstep) in the fabrication process. The second implant species could beused, in some embodiments, to enhance ohmic contact from the sourceand/or drain electrodes of the device and the 2DEG and/or channel layer.In some such embodiments, the implant species may be a n-type dopantsuch as silicon or germanium.

An example of such patterned implantation is shown in FIG. 3A. In FIG.3A, an optional second implantation step can be performed in which asecond species (e.g., high energy boron or nitrogen to damage theIII-nitride crystal lattice and disrupt the 2DEG) has been implantedinto regions 331 of semiconductor structure 200A. In some suchembodiments, the semiconductor structure includes both a first implantedspecies to reduce or eliminate the conductivity of a parasitic channelin patterned or selected regions of the substrate (e.g., in selectedregions of a surface region of the substrate), as well as a secondimplanted species to reduce or eliminate the conductivity of patternedor selected regions of a 2DEG region in the III-nitride material region.The implantation of multiple species in this way may be advantageousaccording to certain, although not necessarily all, embodiments, suchas, for example, in certain cases when implant isolation is used in thefield regions of semiconductor structures to electrically isolate onedevice (e.g., transistor) form another device (e.g., transistor), andwhen implant species are also used to eliminate or reduce the impact ofan existing parasitic channel. Another example illustrating theimplantation of multiple species at different depths is shown in FIGS.3B and 3H. In FIGS. 3B and 3H, an optional second implantation step canbe performed in which a second species has been implanted into regions331 of semiconductor structure 200A.

In certain embodiments, a semiconductor device may comprise a region inwhich the conductivity of a 2DEG region has been reduced or eliminated,for example, by implanting a second species (e.g., boron or nitrogen).In some such embodiments, the semiconductor device may also include aregion—positioned under the region in which the conductivity of the 2DEGregion has been reduced or eliminated—in which the conductivity of aparasitic channel has been reduced or eliminated. For example, referringto FIGS. 3A-3B, in certain embodiments, the semiconductor structure 200may comprise a region 331 in which the conductivity of a 2DEG regionwithin III-nitride material region 120 has been reduced, disrupted, oreliminated (for example, by implanting a second species) and a region330, positioned under region 331, in which the conductivity of aparasitic channel has been reduced (or even eliminated). In some suchembodiments, region 331 has a concentration of both the parasiticspecies (e.g., Ga, Al, etc.) and a concentration of the first implantedspecies (e.g., a species having a relative atomic mass of less than 5).In some embodiments, the semiconductor device may also comprise a regionhaving a high-conductivity 2DEG and a region—positioned under thehigh-conductivity 2DEG region—having a high-conductivity parasiticchannel. For example, referring to FIGS. 3A-3B, in some embodiments, thesemiconductor device may have a region 335 which includes ahigh-conductivity 2DEG, as well as a region 340 formed under region 335which includes a high-conductivity parasitic channel (for example, dueto a relatively high concentration of one or more parasitic species,such as Ga, Al, and the like).

According to certain embodiments, implantation of the second species canreduce the electrical conductivity of the 2DEG by at least about 5%, atleast about 10%, at least about 25%, at least about 50%, at least about75%, at least about 90%, at least about 95%, at least about 99%, ormore, relative to the electrical conductivity of the 2DEG prior to theimplantation of the second species.

The patterned implantation of the second species can be achieved, forexample, using an implantation mask. As shown in FIGS. 3A, 3B, and 3G,the second species is implanted into regions 331 using the sameimplantation mask 300 that is used to pattern the implantation of thefirst species. In other embodiments, the implantation of the secondspecies can be achieved using a second implantation mask that isdifferent from the first implantation mask. For example, in certainembodiments in which the implantation mask shown in FIG. 3C is used toimplant the first species, a different implantation mask may be used toimplant the second species (e.g., to avoid implanting the second specieswithin active regions of the transistor). As one example, in someembodiments, the mask illustrated in FIG. 3C can be used to implant thefirst species and the mask illustrated in FIG. 3F can be used to implantthe second species.

The III-nitride material region into which the second species isimplanted may comprise, in some embodiments, a 2DEG region. In some suchembodiments, the second implanted species can be implanted within the2DEG region of the III-nitride material region. In some suchembodiments, after implantation of the second species into the 2DEGregion, the second implanted species forms a pattern spatially definedacross at least one lateral dimension of the 2DEG region (and, in someembodiments, across two lateral dimensions of the 2DEG region).

In some embodiments, the concentration of the second implanted specieswithin the III-nitride material region is relatively high. For example,in some cases, after the second species is implanted into thesemiconductor structure, the second species is present within at least aportion of the III-nitride material region (e.g., within at least aportion of a 2DEG region of the III-nitride material) at a concentrationof at least about 10¹⁶/cm³ (and/or, in some embodiments, at least about10¹⁷/cm³, at least about 10¹⁸/cm³, at least about 10¹⁹/cm³, at leastabout 10²⁰/cm³, or more). Referring to FIGS. 3A-3B, for example, in someembodiments, after a second species is implanted into III-nitridematerial region 120, the concentration of the second species withinIII-nitride material region 120 (e.g., within region 331 of III-nitridematerial region 120) can be at least about 10¹⁶/cm³. As another example,referring to FIG. 3G, in some embodiments, after a second species isimplanted into III-nitride material region 120, the concentration of thesecond species within surface region 130 (e.g., within region 331 ofIII-nitride material region 120) can be at least about 10¹⁶/cm³.

In some embodiments, after the second species is implanted, at least asecond portion of the III-nitride material region is substantially freeof the second implanted species (i.e., the second implanted species isnot present or is present in an amount of less than about 10¹⁵/cm³, and,in some cases, less than about 10¹⁴/cm³, less than about 10¹³/cm³, lessthan about 10¹²/cm³, less than about 10¹¹/cm³, less than about 10¹⁰/cm³,or lower). Referring to FIGS. 3A-3B, for example, in some embodiments,after a second species is implanted into III-nitride material region120, region 335 of III-nitride material region 120 can be substantiallyfree of the second implanted species. As another example, referring toFIG. 3G, in some embodiments, after a second species is implanted intoIII-nitride material region 120, region 335 of III-nitride materialregion 120 can be substantially free of the second implanted species.The first and second species can be implanted, according to certainembodiments, into a semiconductor structure comprising a transistor(e.g., a FET) located over the substrate. According to some suchembodiments, after the first species is implanted into the semiconductorstructure, at least a portion of the substrate underneath a channelbetween a source of the transistor and a drain of the transistor issubstantially free of the first implanted species (i.e., the firstimplanted species is not present or is present in an amount of less thanabout 10¹⁵/cm³, and, in some cases, less than about 10¹⁴/cm³, less thanabout 10¹³/cm³, less than about 10¹²/cm³, less than about 10¹¹/cm³, lessthan about 10¹⁰/cm³, or lower). For example, referring to FIGS. 3A-3B,in some embodiments, after the first species is implanted (e.g., usingmask 300 shown in FIGS. 3A-3B), regions 335 and 340 can be substantiallyfree of the first implanted species. As another example, referring toFIGS. 3E and 3G, in some embodiments, after the first species isimplanted, regions 335 and 340 can be substantially free of the firstimplanted species. In some such embodiments, after the second species isimplanted, at least a portion of the III-nitride material regionunderneath a channel between a source of the transistor and a drain ofthe transistor is substantially free of the second implanted species(i.e., the second species is not present or is present in an amount ofless than about 10¹⁵/cm³, and, in some cases, less than about 10¹⁴/cm³,less than about 10¹³/cm³, less than about 10¹²/cm³, less than about10¹¹/cm³, less than about 10¹⁰/cm³, or lower). For example, referring toFIGS. 3A-3B, in some embodiments, after the second species is implanted,regions 335 and 340 can be substantially free of the second implantedspecies. As another example, referring to FIGS. 3E and 3G, in someembodiments, after the second species is implanted, regions 335 and 340can be substantially free of the second implanted species.

In some embodiments, the first and/or second species can be implantedbetween first and second transistors in a semiconductor structurecomprising multiple transistors. For example, as noted above, in someembodiments, the semiconductor structure comprises first transistorlocated over the substrate and a second transistor located over thesubstrate and laterally spaced apart from the first transistor. In somesuch embodiments, after the first species is implanted, a portion of thesurface region of the substrate between the first transistor and thesecond transistor has a concentration of the first implanted species ofat least about 10¹⁹/cm³ (or, in some embodiments, at least about10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³, at leastabout 10²³/cm³, or more). For example, referring to FIG. 3J, in someembodiments, after the first species is implanted, region 330 of surfaceregion 130 of substrate 110 between first transistor 210-1 and secondtransistor 210-2 has a concentration of the first implanted species ofat least about 10¹⁹/cm³. In some embodiments, region 330 between secondtransistor 210-2 and third transistor 210-3 and/or region 330 betweenthird transistor 210-3 and fourth transistor 210-4 has a concentrationof the first implanted species of at least about 10¹⁹/cm³.

In some such embodiments, after the second species is implanted, aportion of the III-nitride material region between the first transistorand the second transistor has a concentration of the second implantedspecies of at least about 10¹⁶/cm³ (or, in some embodiments, at leastabout 10¹⁷/cm³, at least about 10¹⁸/cm³, at least about 10¹⁹/cm³, atleast about 10²⁰/cm³, or more). The second species can be implanted,according to some embodiments, such that a portion of a 2DEG regionbetween the first transistor and the second transistor has aconcentration of the second implanted species of at least about 10¹⁶/cm³(or, in some embodiments, at least about 10¹⁷/cm³, at least about10¹⁸/cm³, at least about 10¹⁹/cm³, at least about 10²⁰/cm³, or more).For example, referring to FIG. 3J, in some embodiments, after the secondspecies is implanted, region 331 of III-nitride material region 120between first transistor 210-1 and second transistor 210-2 (which caninclude a portion of a 2DEG region 191 between first transistor 210-1and second transistor 210-2) has a concentration of the second implantedspecies of at least about 10¹⁶/cm³. In some embodiments, region 331between second transistor 210-2 and third transistor 210-3 and/or region331 between third transistor 210-3 and fourth transistor 210-4 (whichcan include portions of a 2DEG region 191 between second transistor210-2 and third transistor 210-3 and/or between third transistor 210-3and fourth transistor 210-4, respectively) has a concentration of thesecond implanted species of at least about 10¹⁶/cm³.

According to certain embodiments, the second species can be implantedsuch that it surrounds at least one transistor of the semiconductorstructure. As one example, in some embodiments, an implantation masksimilar to the mask shown in FIG. 3K can be used to pattern theimplantation of the second species. In some such cases, when the secondspecies is implanted across open region 310, the second implantedspecies can surround each of transistors 210 positioned over substrate110.

Those of ordinary skill in the art, given the guidance provided by theinstant specification, would be capable of selecting appropriateimplantation conditions to achieve a desired implantation pattern. Whenimplanting multiple species at different depths within semiconductorstructures, each implantation step may, in some cases, be performedusing different implant conditions (e.g., dose, energy, and/or implantspecies) may be employed. For example, in some cases, the conditions(e.g., dose, energy, etc.) used to implant the first species (e.g.,having an atomic mass of less than 5) to reduce or eliminate theconductivity of an existing parasitic channel may be different than theconditions (e.g., dose, energy, etc.) used to implant the second species(e.g., a p-type dopant or an n-type dopant) or introduce nitrogenvacancies to reduce or eliminate the conductivity of a 2DEG region of aIII-Nitride material device region). This difference can, in certaincases, be due to (at least in part) the different depths at which theparasitic channel and the 2DEG region are positioned. For example, theparasitic channel may be located at a relatively deep location (e.g.,2-5 microns below surface 135 of the substrate, or more) while the 2DEGregion may be relatively shallow.

According to certain embodiments, patterned implantation of the firstand/or second species may result in the formation of at least one p-njunction defined laterally across a portion of the semiconductorstructure.

For example, patterned implantation of the first species into a surfaceregion of the substrate may form, according to certain embodiments, ap-n junction defined laterally across the surface region of thesubstrate. Referring to FIG. 3A, surface region 130 of substrate 110 mayhave an original doping profile (e.g., n-type or p-type). In some cases,formation of III-nitride material region 120 may result in the diffusionof one or more dopants (e.g., Ga, In, As, N, and the like) intosubstrate 110, causing the doping type of surface region 130 to change(e.g., from an n-type region to a p-type region, or from a p-type regionto an n-type region). In some embodiments, patterned implantation of thefirst species into regions 330 of surface region 130 can cause thedoping type of regions 330 to change again (e.g., from n-type to p-type,or from p-type to n-type). In some such embodiments, region 340, becauseit is not exposed to the first implanted species, retains the dopingtype present after diffusion of the parasitic species into surfaceregion 130. In some such embodiments, regions 330 form p-n junctionswith region 340.

As another example, patterned implantation of the second species into aIII-nitride material region (e.g., into a 2DEG region of a III-nitridematerial region) may form, according to certain embodiments, a p-njunction defined laterally across the III-nitride material region (e.g.,defined laterally across a 2DEG region within a III-nitride materialregion) of the semiconductor structure. Referring to FIG. 3A, forexample, III-nitride material region 120 may have an original dopingprofile (e.g., n-type or p-type). In some embodiments, patternedimplantation of the second species into regions 331 of III-nitridematerial region 120 can cause the doping types of regions 331 to change(e.g., from p-type to n-type, or from n-type to p-type). In some suchembodiments, region 335, because it is not exposed to the secondimplanted species, retains the doping type present after formation ofthe III-nitride material region. In some such embodiments, regions 331form p-n junctions with region 335.

According to certain embodiments, one or more counter-dopants can beused to counteract the electronic effect of a dopant within theparasitic region of the semiconductor structure. The counter-doping maybe performed, according to some such embodiments, to reduce the freecarrier concentration within the parasitic region of the semiconductorstructure.

Accordingly, certain embodiments comprise implanting a counter-dopantinto the semiconductor structure such that a concentration profile ofthe counter-dopant substantially matches a concentration profile of asecond dopant present within the substrate. Generally, the concentrationprofile of a first dopant type matches the concentration profile of asecond dopant type within a region (e.g., within a substrate) when boththe first and second dopant types are present in the substrate, and themaximum free carrier concentration through the thickness of the region(e.g., through the thickness of the substrate) is less than about10¹⁷/cm³. In some embodiments, a concentration profile of a first dopanttype matches the concentration profile of a second dopant type within aregion (e.g., within a substrate) such that the maximum free carrierconcentration through the thickness of the region (e.g., through thethickness of the substrate) is less than about 10¹⁶/cm³, less than about10¹⁵/cm³, less than about 10¹⁴/cm³, less than about 10¹³/cm³, less thanabout 10¹²/cm³, less than about 10¹¹/cm³, or less than about 10¹⁰/cm³.One of ordinary skill in the art would understand that, when determiningthe n-type dopant concentration profile, the contributions of all n-typedopants are considered as a whole. Thus, when multiple n-type dopantsare present, the n-type dopant concentration profile corresponds to theprofile generated when summing the concentrations of all n-type dopantsthrough the thickness of the substrate. Similarly, when determining thep-type dopant concentration profile, the contributions of all p-typedopants are considered as a whole. Thus, when multiple p-type dopantsare present, the p-type dopant concentration profile corresponds to theprofile generated when summing the concentrations of all p-type dopantsthrough the thickness of the substrate.

The counter-dopant and the second dopant originally present in theparasitic channel of the substrate can be of the opposite type. Forexample, in some embodiments, the parasitic channel comprises a p-typedopant (e.g., Al and/or Ga), and the counter-dopant comprises an n-typedopant (e.g., As and/or P). In certain embodiments, the parasiticchannel comprises an n-type dopant (e.g., As and/or P), and thecounter-dopant comprises a p-type dopant (e.g., Al and/or Ga).

Those of ordinary skill in the art are familiar with a variety ofsystems and methods that can be used to perform counter-doping. Forexample, counter-doping may be performed by growing a counter-dopedlayer (e.g., a counter-doped epitaxial layer) over the substrate (e.g.,ex-situ). Counter-doping may also be performed, for example, byimplanting (or diffusing) a counter-dopant into a surface of thesubstrate (e.g., surface 135 of substrate 110 in the figures). As yetanother example, counter-doping may be performed by implanting acounter-dopant using high energies (e.g., such that the surface regionof the substrate becomes non-conductive).

Certain embodiments are related to semiconductor structures comprisingsubstrates with multiple dopants whose concentration profiles aresubstantially matched. Such structures may be formed, for example, usingthe counter-doping methods described above. According to certainembodiments, the substrate of the semiconductor structure (e.g., any ofthe semiconductor structures described elsewhere herein) comprises ap-type dopant having a first concentration profile, and an n-type dopanthaving a second concentration profile that is substantially matched tothe first concentration profile. The n-type dopant may be, for example,As and/or P. The p-type dopant may be, for example, Al and/or Ga.

According to certain embodiments in which counter-dopant(s) areimplanted into the substrate, the substrate may also include arelatively high peak concentration of Group III species (e.g., Al, Ga,In, Tl, and B). The relatively high peak concentration(s) of Group IIIspecies can be present before the implantation step, according to someembodiments. In certain embodiments, the relatively high peakconcentration(s) of Group III species can be present after theimplantation step. In some embodiments, the presence of thecounter-dopant(s) can reduce the electronic conductivity of the GroupIII species, inhibiting the formation of a high conductivity parasiticchannel. In some embodiments in which counter-dopant(s) are implantedinto the substrate, the peak of the sum of the concentrations of GroupIII species in the substrate (e.g., the peak of the sum of theconcentrations of Al, Ga, In, Tl, and B in the substrate) is at leastabout 10¹⁷/cm³, at least about 10¹⁸/cm³, at least about 10¹⁹/cm³, atleast about 10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³,or at least about 10²³/cm³. In some embodiments in whichcounter-dopant(s) are implanted into the substrate, the peak of the sumof the concentrations of Al, Ga, and In in the substrate is at leastabout 10¹⁷/cm³, at least about 10¹⁸/cm³, at least about 10¹⁹/cm³, atleast about 10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³,or at least about 10²³/cm³. In some embodiments in whichcounter-dopant(s) are implanted into the substrate, the peakconcentration of Al, Ga, and/or In in the substrate is at least about10¹⁷/cm³, at least about 10¹⁸/cm³, at least about 10¹⁹/cm³, at leastabout 10²⁰/cm³, at least about 10²¹/cm³, at least about 10²²/cm³, or atleast about 10²³/cm³.

In some embodiments, the substrate comprises a high-conductivityparasitic channel before the implantation of counter-dopant(s) and alow-conductivity parasitic channel (or no parasitic channel) after theimplantation of counter-dopant(s). In some embodiments, the bulk regionof the substrate has a lower peak free carrier concentration than thetop surface region of the substrate before and/or after the implantationof counter dopant(s). In some embodiments, the bulk region of thesubstrate is doped with a first free carrier type and the top surfaceregion is doped with a second free carrier type (e.g., a Group IIIspecies such as Al, Ga, In, and/or Tl) before and/or after theimplantation of counter-dopant(s). According to certain embodiments, thepeak free carrier concentration in the bulk region is less than about10¹³/cm³, less than about 10¹²/cm³, less than about 10¹¹/cm³, less thanabout 10¹⁰/cm³, or less than about 10⁹/cm³ before and/or after theimplantation of counter-dopant(s).

According to certain embodiments, one or more active species capable ofreacting with a species originating in a location external to thesubstrate (also referred to herein as an “external species”) isincorporated into the substrate (e.g., into a surface region of asubstrate, which can be all or part of a silicon layer). The activespecies within the substrate can inhibit (or prevent) the formation ofparasitic channels within the substrate (e.g., within the surface regionof the substrate) by, for example, reacting with one or more externalspecies that diffuses into or is otherwise transported into thesubstrate (e.g., during growth of the III-nitride material region). Insome such embodiments, the presence of the active species ensures thatthe peak free carrier concentration within the substrate (e.g., withinthe surface region of the substrate) remains lower than it otherwisewould be in the absence of the active species but under otherwiseidentical conditions.

Accordingly, certain inventive methods comprise forming a III-nitridematerial region over a surface of a substrate such that an activespecies within the substrate (e.g., within a surface region of thesubstrate) reacts with at least a portion of an external species thatcontacts the substrate during the formation of the III-nitride materialregion. For example, referring to FIGS. 1A-1B, in some embodiments,during at least a portion of the time during which at least a portion ofIII-nitride material region 120 is formed, an active species withinsubstrate 110 (e.g., within surface region 130 of substrate 110) mayreact with a species external to substrate 110. In some suchembodiments, the free carrier concentration within the parasitic channelwithin surface region 130 may remain relatively low. In certainembodiments, the parasitic channel within surface region 130 may even becompletely eliminated from substrate 110.

A variety of active species capable of reacting with the speciesoriginating from a location external to the substrate can be used.Generally, such species are non-silicon species. The active species maybe molecular or atomic, and may be charged or uncharged. In someembodiments, the species capable of reacting with the external speciescomprises oxygen (e.g., O₂, O⁺), nitrogen (e.g., N₂), carbon, copper(e.g., in metallic form), and/or iron (e.g., in metallic form).According to certain embodiments, the species capable of reacting withthe external species comprises O⁺. In some embodiments, the activespecies capable of reacting with the external species comprises oxygen,and the oxygen oxidizes an external species that diffuses into thesubstrate during at least a portion of the time during which theIII-nitride material region is formed.

The active species capable of reacting with a species external to thesubstrate can be, in some embodiments, formed in the substrate prior tothe III-nitride material region growth step. Suitable processes forforming such regions in the substrate are known to those of ordinaryskill in the art. For example, in some embodiments, the active speciescan be formed in the substrate by growing a layer comprising the activespecies over the substrate (e.g., ex-situ). In certain embodiments, theactive species can be formed in the substrate by implanting (ordiffusing) the active species into a surface of the substrate (e.g.,surface 135 of substrate 110 in the figures). As yet another example,the active species can be formed in the substrate by implanting theactive species using high energies.

The species external to the substrate (with which the active speciesreacts) may arise from a variety of sources. In some cases, the speciesexternal to the substrate may accumulate on the substrate surface (or anoverlying layer/region), for example, after being introduced into areaction chamber in which the substrate is placed, but prior toformation of the III-nitride material region on the substrate. Forexample, the species external to the substrate can be a contaminantoriginating from a reactant fed to the reactor. The species external tothe substrate can also be a contaminant originating from a non-reactantsource outside the reactor. For example, in some embodiments, thespecies external to the substrate may accumulate on the substrate (or anoverlying layer) before the substrate is introduced into the reactor andmay accompany the substrate as the substrate is introduced into thereactor. In certain cases, the species external to the substrate may bepresent within the reactor before the substrate is inserted into thereactor. For example, in some cases, the species external to thesubstrate may be a species that forms a part of the reactor (e.g., ametal such as Fe, Ni, and/or Cr) in which the III-nitride materialregion is grown. As another example, in some cases, the species externalto the substrate may be a contaminant within the reactor that waspresent prior to insertion of the substrate into the reactor. In stillfurther cases, the species external to the substrate can be a residualreactant left over from a previous reaction process. In certainembodiments, the external species is all or part of a precursor of theIII-nitride material. In some embodiments, the species external to thesubstrate is an organic species (e.g., an organic component of areaction precursor of the III-nitride material). The species external tothe substrate could also be, in some embodiments, a group III element(e.g., Ga, In, and/or Al). Combinations of any or all of the abovespecies external to the substrate are also possible.

Certain embodiments are related to semiconductor structures comprisingsubstrates containing at least one active species. The active species,when present in the substrate (e.g., a surface region of the substrate),can be coupled with an external species (i.e., coupled with a speciesthat originated from outside the substrate) or can be capable ofreacting with a species external to the substrate (e.g., any of theexternal species mentioned above). In some embodiments, theconcentration, within the substrate (e.g., within the surface region ofthe substrate), of the active species coupled to the external species orcapable of reacting with the external species is at least about at leastabout 10¹⁹/cm³ (or at least about 10²⁰/cm³, at least about 10²¹/cm³, atleast about 10²²/cm³, at least about 10²³/cm³, or more). Theconcentration of active species (either coupled to an external speciesor capable of reacting with an external species) can be measured usingstandard techniques known to those of ordinary skill in the artincluding Secondary Ion Mass Spectroscopy (SIMS).

According to certain embodiments in which active species are included inthe substrate, the substrate may also include a relatively low peakconcentration of Group III species (e.g., Al, Ga, In, Tl, and B), forexample, after growth of the III-nitride region and/or after formationof the transistor. In some such embodiments, the one or more activespecies can react with Group III species, inhibiting the buildup ofGroup III species in the substrate. In some embodiments in which activespecies are included in the substrate, the peak of the sum of theconcentrations of Group III species in the substrate (e.g., the peak ofthe sum of the concentrations of Al, Ga, In, Tl, and B in the substrate)is less than about 10¹⁷/cm³, less than about 10¹⁶/cm³, less than about10¹⁵/cm³, less than about 10¹⁴/cm³, less than about 10¹³/cm³, less thanabout 10¹²/cm³, less than about 10¹¹/cm³, or less than about 10¹⁰/cm³(e.g., after the formation of the III-nitride region and/or afterformation of the transistor). In some embodiments in which activespecies are included in the substrate, the peak of the sum of theconcentrations of Al, Ga, and In in the substrate is less than about10¹⁷/cm³, less than about 10¹⁶/cm³, less than about 10¹⁵/cm³, less thanabout 10¹⁴/cm³, less than about 10¹³/cm³, less than about 10¹²/cm³, lessthan about 10¹¹/cm³, or less than about 10¹⁰/cm³ (e.g., after formationof the III-nitride region and/or after formation of the transistor). Insome embodiments in which active species are included in the substrate,the peak concentration of Al, Ga, and/or In in the substrate is lessthan about 10¹⁷/cm³, less than about 10¹⁶/cm³, less than about 10¹⁵/cm³,less than about 10¹⁴/cm³, less than about 10¹³/cm³, less than about10¹²/cm³, less than about 10¹¹/cm³, or less than about 10¹⁰/cm³ (e.g.,after formation of the III-nitride region and/or after formation of thetransistor).

As noted above, certain embodiments relate to methods (and associatedstructures) in which diffusion barrier regions are used to inhibit orprevent the diffusion of material that increases the conductivity of thesubstrate (which increased conductivity can lead to the formation ofhigh-conductivity parasitic channels). Dopants and/or other species cangenerally diffuse into the substrate after accumulating on the substratesurface, or on the surface of a layer overlying the substrate throughwhich the dopants also diffuse. Thus, as described further below,certain methods can include forming one or more layers that inhibits orprevents the diffusion of dopants or other species into the substrate.

According to certain embodiments in which diffusion barrier regions areincluded in the semiconductor structure, the substrate may also includea relatively low peak concentration of Group III species (e.g., Al, Ga,In, Tl, and B), for example, after growth of the III-nitride regionand/or after formation of the transistor. In some such embodiments, thediffusion barrier region can inhibit or prevent the diffusion of GroupIII species into the substrate, inhibiting the buildup of Group IIIspecies in the substrate.

In some embodiments in which in which diffusion barrier regions areincluded in the semiconductor structure, the peak of the sum of theconcentrations of Group III species in the substrate (e.g., the peak ofthe sum of the concentrations of Al, Ga, In, Tl, and B in the substrate)is less than about 10¹⁷/cm³, less than about 10¹⁶/cm³, less than about10¹⁵/cm³, less than about 10¹⁴/cm³, less than about 10¹³/cm³, less thanabout 10¹²/cm³, less than about 10¹¹/cm³, or less than about 10¹⁰/cm³(e.g., after formation of the III-nitride region and/or after formationof the transistor). In some embodiments in which diffusion barrierregions are included in the semiconductor structure, the peak of the sumof the concentrations of Al, Ga, and In in the substrate is less thanabout 10¹⁷/cm³, less than about 10¹⁶/cm³, less than about 10¹⁵/cm³, lessthan about 10¹⁴/cm³, less than about 10¹³/cm³, less than about 10¹²/cm³,less than about 10¹¹/cm³, or less than about 10¹⁰/cm³ (e.g., afterformation of the III-nitride region and/or after formation of thetransistor). In some embodiments in which diffusion barrier regions areincluded in the semiconductor structure, the peak concentration of Al,Ga, and/or In in the substrate is less than about 10¹⁷/cm³, less thanabout 10¹⁶/cm³, less than about 10¹⁵/cm³, less than about 10¹⁴/cm³, lessthan about 10¹³/cm³, less than about 10¹²/cm³, less than about 10¹¹/cm³,or less than about 10¹⁰/cm³ (e.g., after formation of the III-nitrideregion and/or after formation of the transistor).

According to certain embodiments, the diffusion barrier can inhibit orprevent the formation of a parasitic channel in the substrate during asubsequent step of forming a III-nitride material region over thesubstrate. For example, some embodiments comprise forming a III-nitridematerial region over a substrate over which a diffusion barrier regionhas been positioned (e.g., formed). According to some such embodiments,after forming the III-nitride material region over the diffusion barrierregion, the surface region of the substrate over which the III-nitridematerial is formed comprises a low-conductivity parasitic channel or thesurface region (and/or the remainder of the substrate) is free of aparasitic channel. This can lead to the formation of semiconductordevices comprising III-nitride material regions positioned oversubstrates in which the substrate has no parasitic channel or includesonly a low-conductivity parasitic channel (e.g., in the surface regionover which the III-nitride material region is formed). For example,referring to FIG. 1B, in some embodiments, after forming III-nitridematerial region 120 over diffusion barrier region 140, surface region130 of substrate 110 over which III-nitride material region 120 isformed comprises a low-conductivity parasitic channel or surface region130 (and/or the remainder of substrate 110) is free of a parasiticchannel.

The diffusion barrier region can comprise, in some embodiments, asingle-crystal diffusion barrier layer. In some embodiments, thediffusion barrier region comprises an epitaxial layer.

According to certain embodiments, the diffusion barrier region comprisesan AlN region. In some such embodiments, the diffusion barrier comprisesa low-temperature AlN region located over the substrate and ahigh-temperature AlN region located over the substrate. For example, inthe embodiment illustrated in FIG. 1C, diffusion barrier region 140comprises low-temperature AlN region 140A (which can be positioned oversubstrate 110). In addition, diffusion barrier region 140 compriseshigh-temperature AlN region 140B (which can be positioned over substrate110 and over low-temperature AlN region 140A). In addition, as shown inFIG. 1B, semiconductor structure 100B comprises III-nitride materialregion 120, which can be located over low-temperature AlN region 140Aand over high-temperature AlN region 140B when diffusion barrier region140 includes such layers. While a multi-layer AlN diffusion barrierregion is illustrated and described in association with FIG. 1C, itshould be understood that such diffusion barrier regions could be usedin any of diffusion barrier regions 140 shown in the figures.

Those of ordinary skill in the art would be capable of determiningwhether a particular AlN region is formed at low temperatures (e.g.,temperatures below about 950° C.) or high temperatures (e.g.,temperatures of about 950° C. or higher). Low-temperature AlN regionstypically exhibit a relatively rough morphology, often including athree-dimensional surface structure. Such low-temperature AlN regionscan include protrusions that are triangular and/or columnar incross-section. High-temperature AlN regions, on the other hand,generally have a smoother morphology and develop a two-dimensionalstep-wise surface. According to certain embodiments, the low-temperatureAlN region can be formed first, after which the high-temperature AlNregion can be formed. In some such embodiments, the low-temperature AlNregion is unsuitable for gallium nitride material growth, and thehigh-temperature AlN region can be used to provide a suitable substratefor subsequent gallium nitride material growth. Another artifact ofusing a low temperature and high temperature AlN nucleation bilayer is achange in impurity levels found within the two layers, for example thecarbon impurity concentration. Typically, higher growth temperature AlNlayers contain lower levels of carbon impurities, and lower growthtemperature AlN contain higher levels of carbon impurities. Therefore,in certain cases, if two different growth temperatures were used for theAlN layers, there would be two distinct levels of carbon impuritiespresent. Accordingly, in some embodiments, the semiconductor structurecomprises a first AlN layer with a first level of impurities (e.g.,carbon impurities) and a second AlN layer with a second level ofimpurities (e.g., carbon impurities) different from the first level ofimpurities (e.g., at least about 2 at % different, at least about 5 at %different, or more, relative to the level of impurities in the first AlNlayer).

Certain embodiments relate to inventive methods for formingsemiconductor structures comprising multiple AlN regions. According tocertain embodiments, a first AlN region is formed over a substrate,wherein the temperature of the environment in which the first AlN regionis formed is below about 950° C. (e.g., between about 700° C. and about950° C.). Some embodiments comprise forming a second AlN region over thesubstrate, wherein the temperature of the environment in which thesecond AlN region is formed is at least about 950° C. (e.g., from about950° C. to about 1150° C.). Certain such embodiments comprise forming aIII-nitride material region over the first AlN region and over thesecond AlN material region.

In some embodiments, the low-temperature AlN region is formed first andthe high-temperature AlN region is formed after the low-temperature AlNregion is formed. For example, referring to FIG. 1C, in someembodiments, low-temperature AlN region 140A is formed over substrate110, and subsequently, high-temperature AlN region 140B is formed oversubstrate 110 and over low-temperature AlN region 140A. In some suchembodiments, the III-nitride material region is then formed. Forexample, in FIG. 1C, III-nitride material region 120 can be formed oversubstrate 110 after low-temperature AlN region 140A and high-temperatureAlN region 140B have been formed over substrate 110.

In certain embodiments, the high-temperature AlN region is formed firstand the low-temperature AlN region is formed after the high-temperatureAlN region is formed. In such embodiments, the high-temperature AlNregion can be positioned over the substrate and the low-temperature AlNregion can be positioned over both the substrate and thehigh-temperature AlN region.

In certain embodiments, the device comprises an amorphous or non-singlecrystalline silicon nitride-based or aluminum-nitride based layerbetween the substrate and the III-nitride material nucleation layer. Insome such embodiments, the amorphous or non-single crystalline layer maybe formed of Si_(x)N_(y) and/or Al_(x)Si_((1-x))N, and may optionallycomprise one or more additional elements (e.g., oxygen). The amorphousor non-single crystalline layer may be relatively thin, according tosome embodiments. For example, in some embodiments, the amorphous ornon-single crystalline layer may have a thickness of less than about 30Angstroms, less than about 20 Angstroms, or less than about 10 Angstroms(and/or, in some embodiments, down to 5 Angstroms thick, or less). Theuse of a relatively thin layer can allow one to avoid destroying theepitaxial template used to perform heteroepitaxial III-nitride materialgrowth.

In some embodiments, the amorphous or non-single crystalline siliconnitride-based or aluminum-nitride based layer can be amorphous. Incertain embodiments, the amorphous or non-single crystalline siliconnitride-based or aluminum-nitride based layer can be formed directly onthe substrate (e.g., a silicon portion of the substrate). In some suchembodiments, the amorphous or non-single crystalline siliconnitride-based or aluminum-nitride based layer may be formed bynitridating a top surface of the silicon substrate. In a nitridationprocess, nitrogen reacts with a top surface region of the siliconsubstrate to form a silicon nitride-based layer. The top surface may benitridated by exposing the silicon substrate to a gaseous source ofnitrogen at elevated temperatures. For example, ammonia may beintroduced into a process chamber in which a silicon substrate ispositioned. The temperature in the process chamber may be between about1000° C. and about 1100° C. and the pressure may be between about 20torr and about 40 torr (in some cases, about 30 torr). The reactionbetween nitrogen and the silicon substrate can be allowed to proceed fora reaction time selected to produce a layer having a desired thickness.It should be understood that other processes may be used to form siliconnitride-based layers including processes (e.g., CVD processes) that useseparate nitrogen and silicon sources.

According to certain embodiments, the diffusion barrier region comprisesa layer comprising a rare-earth oxide and/or a rare-earth nitride. Thecategory “rare-earth oxide and/or rare-earth nitride” includesrare-earth oxides, rare-earth nitrides, and combination rare-earthoxides and rare-earth nitrides (e.g., rare-earth oxynitrides). In someembodiments, referring to FIG. 1B, diffusion barrier region 140 cancomprise, a layer comprising a rare-earth oxide and/or a rare-earthnitride.

Some embodiments are related to methods of forming semiconductorstructures comprising layers comprising a rare-earth oxide and/or arare-earth nitride. For example, in some embodiments, a layer comprisinga rare-earth oxide and/or a rare-earth nitride is formed over asubstrate. In some such embodiments, a III-nitride material region isformed over the layer comprising the a rare-earth oxide and/or arare-earth nitride. Referring to FIG. 1B, for example, in someembodiments, a layer comprising a rare-earth oxide and/or a rare-earthnitride is formed as part (or all) of diffusion barrier region 140. Insome such embodiments, III-nitride material region 120 is subsequentlyformed over diffusion barrier region 140.

While a diffusion barrier region containing a rare-earth oxide and/or arare-earth nitride is illustrated and described in association with FIG.1B, it should be understood that such diffusion barrier regions could beused in any of diffusion barrier regions 140 shown in the figures.

A variety of types of rare-earth oxides and/or rare-earth nitrides canbe used in the diffusion barrier region. As used herein, a “rare-earth”element refers to an element selected from the group consisting ofcerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium(Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd),praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc),terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y).

A “rare-earth oxide” is any oxide species containing at least onerare-earth element. In some embodiments, the rare-earth oxide contains asingle rare-earth element. In other embodiments, the rare-earth oxidecontains two or more rare-earth elements. In some embodiments, thediffusion barrier region comprises an erbium oxide (e.g., Er₂O₃), agadolinium oxide (e.g., Gd₂O₃), a cerium oxide (e.g., CeO₂), and/or ayttrium oxide (e.g., Y₂O₃). In some embodiments, the rare-earth oxide isfree of nitrogen. In certain embodiments, the rare-earth oxide is not arare-earth oxynitride.

A “rare-earth nitride” is any nitride species containing at least onerare-earth element. In some embodiments, the rare-earth nitride containsa single rare-earth element. In other embodiments, the rare-earthnitride contains two or more rare-earth elements. In some embodiments,the diffusion barrier region comprises an erbium nitride (e.g., Er₂N), agadolinium nitride (e.g., GdN), a cerium nitride (e.g., CeN), and/or ayttrium nitride (e.g., YN). In some embodiments, the rare-earth nitrideis free of oxygen. In certain embodiments, the rare-earth nitride is nota rare-earth oxynitride.

A “rare-earth oxynitride” is a compound in which both oxygen andnitrogen are present, along with at least one rare-earth element. Arare-earth oxynitride is considered to be both a rare-earth oxide and arare-earth nitride. In some embodiments, the rare-earth oxynitridecontains a single rare-earth element. In other embodiments, therare-earth oxynitride contains two or more rare-earth elements. In someembodiments, the diffusion barrier region comprises an erbiumoxynitride, a gadolinium oxynitride, a cerium oxynitride, and/or ayttrium oxynitride.

In certain embodiments, the amount(s) of the rare-earth element(s) ofthe rare-earth oxide and/or rare-earth nitride may be graded across thethickness of the rare-earth oxide and/or rare earth nitride in variousgrading schemes (e.g., compositionally graded, step-wise graded,continuously or discontinuously graded). According to some embodiments,the amount(s) of oxygen and/or nitrogen may be graded across thethickness of the rare-earth oxide and/or rare-earth nitride in variousgrading schemes (e.g., compositionally graded, step-wise graded,continuously or discontinuously graded). In some embodiments, both theamount(s) of rare-earth element(s) as well as the amount(s) of oxygenand/or nitrogen may be graded in various grading schemes (e.g.,compositionally graded, step-wise graded, continuously ordiscontinuously graded).

According to certain embodiments, the diffusion barrier regioncontaining the rare-earth oxide and/or rare-earth nitride can be made upof a relatively high percentage of rare-earth oxide and/or rare-earthnitride material. For example, in some embodiments, at least about 80weight percent (wt %), at least about 90 wt %, at least about 95 wt %,at least about 98 wt %, at least about 99 wt %, or at least about 99.9wt % of the diffusion barrier region is made up of rare-earth oxideand/or rare-earth nitride material. In some embodiments, at least about80 wt %, at least about 90 wt %, at least about 95 wt %, at least about98 wt %, at least about 99 wt %, or at least about 99.9 wt % of thediffusion barrier region is made up of rare-earth oxide material. Insome embodiments, at least about 80 wt %, at least about 90 wt %, atleast about 95 wt %, at least about 98 wt %, at least about 99 wt %, orat least about 99.9 wt % of the diffusion barrier region is made up ofrare-earth nitride material. In some embodiments, at least about 80 wt%, at least about 90 wt %, at least about 95 wt %, at least about 98 wt%, at least about 99 wt %, or at least about 99.9 wt % of the diffusionbarrier region is made up of rare-earth oxynitride material.

In some embodiments, diffusion barrier region 140 may comprise multiplelayers of rare-earth oxide and/or rare-earth nitride material. Forexample, diffusion barrier region 140 may comprise at least onerare-earth oxide containing layer and at least one rare-earth nitridecontaining layer. In some embodiments, diffusion barrier region 140 maycomprise at least two rare-earth oxide containing layers and/or at leasttwo rare-earth nitride containing layers.

In some embodiments in which the diffusion barrier region comprises arare-earth oxide and/or rare-earth nitride material, the diffusionbarrier region comprising the rare-earth oxide and/or rare-earth nitridecan be relatively thin. For example, in some embodiments, the diffusionbarrier region comprising the rare-earth oxide and/or rare-earth nitride(either in the form of a single layer or in the form of multiple layers)can have a thickness of less than about 500 nm, less than about 200 nm,less than about 100 nm, less than about 50 nm, less than about 20 nm, orless than about 10 nm (and/or, in some embodiments, as thin as 5 nm, asthin as 1 nm, or thinner).

According to certain embodiments, the diffusion barrier region comprisesa silicon carbide region. For example, referring to FIG. 1B, diffusionbarrier region 140 can comprise, according to some embodiments, a layercomprising silicon carbide.

Some embodiments are related to methods of forming semiconductorstructures comprising layers comprising silicon carbide. For example, insome embodiments, a silicon carbide region is formed over a substrate.In some such embodiments, a III-nitride material region is formed overthe silicon carbide region and the substrate.

While diffusion barrier regions comprising silicon carbide regions areillustrated and described in association with FIG. 1B, it should beunderstood that such diffusion barrier regions could be used in any ofdiffusion barrier regions 140 shown in the figures.

In some embodiments, the silicon carbide region comprises an epitaxiallayer of 3C—SiC (e.g., formed on the surface of the substrate). Incertain embodiments, a silicon carbide region is formed via reactionand/or consumption of a silicon surface of a silicon substrate with acarbon species. For example, in some embodiments, a silicon carbideregion is formed by contacting carbon with a near surface layer of asilicon substrate, and carbonizing or carburizing the near surface layerof a silicon substrate. In some embodiments, a silicon carbide regionmay be formed of two or more layers of 3C—SiC formed using differentgrowth techniques. For example, in one embodiments, the silicon carbideregion comprises a thin carbonized layer formed on the silicon surface,followed by a thicker epitaxially grown 3C—SiC layer above thecarbonized layer. In some embodiments, the two or more layers of siliconcarbide may include other types of silicon carbide, not limited to3C—SiC polytypes. For example, the silicon carbide region can comprise,in some embodiments, a layer of 4H—SiC, 6H—SiC, or another siliconcarbide polytype.

In certain embodiments, the silicon carbide containing diffusion barrierregion can be relatively thick. For example, in some embodiments, thesilicon carbide region can be greater than about 0.1 microns thick,greater than about 1 micron thick, or greater and about 2 microns thick(and/or, in some embodiments, up to 3 microns thick, or thicker). Suchlarge thicknesses can be used, according to certain embodiments, due tothe relatively high thermal conductivity of silicon carbide.

In certain embodiments, the diffusion barrier region may comprise atleast one doped 3C—SiC layer. For example, epitaxial 3C—SiC as depositedmay have a relatively low resistivity (for example, about 100 ohm-cm orless). In certain embodiments, it may be desired to use a highlyresistivity 3C—SiC diffusion barrier. Certain embodiments comprisedoping the 3C—SiC (e.g., with vanadium dopants). Doping the 3C—SiC mayincrease the resistivity of the 3C—SiC layer, for example, to greaterthan about 100 ohm-cm, greater than about 1000 ohm-cm, or greater thanabout 10,000 ohm-cm. In certain embodiments, it may be desired to us aconductive substrate. In some such cases, N- or P-type dopants may beadded to the 3C—SiC layer(s) to increase electronic conductivity and/orto create a P-N junction within the 3C—SiC diffusion barrier.

According to certain embodiments, the diffusion barrier region comprisesan elemental diboride diffusion barrier layer. For example, referring toFIG. 1B, diffusion barrier region 140 can comprise, according to someembodiments, a layer comprising an elemental diboride. “Elementaldiboride,” as used herein, refers to compounds having the formula(X)₁B₂, wherein B is boron and X is any other element or combination ofelements. The formula (X)₁B₂ is used to indicate that the stoichiometricratio of the total number of atoms of X elements in the compound(whether a single X element is present, or more than one X element ispresent) to the number of boron atoms in the compound is 1:2. In someembodiments, X can be a metal or combination of metals. In someembodiments, X can comprise a transition metal (or a combination oftransition metals). In some embodiments, X can be selected from thegroup consisting of aluminum (Al), zirconium (Zr), Hafnium (Hf), andcombinations and/or alloys of these. Examples of such compounds includeZrB₂, HfB₂, AlB₂, Hf_((x))Zr_((1-x))B₂, Hf_((x))Al_((1-x))B₂,Al_((x))Zr_((1-x))B₂, and Al_((x))Hf_((y))Zr_((1-x-y))B₂. In someembodiments, the elemental diboride comprises zirconium diboride (ZrB₂).

The elemental diboride layer can be, in some embodiments, asingle-crystal elemental diboride layer. In some embodiments, theelemental diboride layer can be an epitaxial layer. In some embodiments,impurity atoms may be present within the elemental diboride layer (e.g.,in interstitial spaces within the elemental diboride lattice).

Some embodiments are related to methods of forming semiconductorstructures comprising diffusion barrier regions comprising an elementaldiboride material layer. For example, in some embodiments, a diffusionbarrier region comprising an elemental diboride material layer (e.g., azirconium diboride (ZrB₂) material layer or any of the other elementaldiboride materials described herein) is formed over a substrate. In somesuch embodiments, a III-nitride material region is formed over thediffusion barrier region comprising the elemental diboride.

While barrier regions comprising an elemental diboride are illustratedand described in association with FIG. 1B, it should be understood thatsuch barrier regions could be used in any of barrier regions 140 shownin the figures.

In some embodiments in which the diffusion barrier region comprises anelemental diboride material, the diffusion barrier region comprising theelemental diboride can be relatively thin. For example, in someembodiments, the diffusion barrier region comprising the elementaldiboride can have a thickness of less than about 500 nm, less than about200 nm, less than about 100 nm, less than about 50 nm, or less thanabout 25 nm (and/or, in some embodiments, as thin as 15 nm, as thin as10 nm, or thinner).

In certain embodiments in which the diffusion barrier region 140comprises an elemental diboride, it may be (although is not necessarily)advantageous to use a miscut silicon substrate. Using the miscut siliconsubstrate can, according to certain embodiments, aid in the quality ofthe epitaxial elemental diboride layer according to some suchembodiments.

According to certain embodiments, two or more of the parasitic channelmitigation/prevention strategies described herein can be used incombination. For example, in some embodiments, multiple diffusionbarrier layers (e.g., two or more of AlN, rare-earth oxide and/orrare-earth nitride, silicon carbide, and elemental diboride) can beemployed in combination, with one or more types of diffusion barriermaterials within each diffusion barrier layer. In some embodiments, oneor more diffusion barriers may be employed in combination with theimplantation of a species capable of reacting with an external species,implantation of a species having an atomic mass of less than 5, and/orcounter-doping of one or more species. In addition, the implantation ofa species capable of reacting with an external species can be used incombination with the implantation of a species having an atomic mass ofless than 5, in combination with counter-doping of one or more species,and/or in combination with the use of one or more diffusion barriers.According to certain embodiments, the implantation of a species havingan atomic mass of less than 5 can be used in combination with theimplantation of a species capable of reacting with an external species,the counter-doping of one or more species, and/or in combination withthe use of one or more diffusion barriers. In some embodiments,counter-doping of one or more species can be used in combination withthe implantation of a species having an atomic mass of less than 5, theimplantation of a species capable of reacting with an external species,and/or in combination with the use of one or more diffusion barriers.Patterned implantation, at one or multiple depths, may also be used incombination with any of these strategies.

Various of the devices described herein may be made using conventionalsemiconductor processing techniques. Such processing techniques caninvolve, for example, growing layers on the substrate in a processchamber under vacuum conditions.

Some methods may include cleaning the substrate surface prior to growingoverlying layers and, typically, before introduction into the processchamber. The substrate surface may be cleaned to remove residual dopantspecies that may diffuse into the substrate during processing. Forexample, the substrate may be cleaned by wet chemical cleaning agentssuch as buffered oxide etch (BOE), hydro-fluoric acid (HF), RCA clean(which is a commercial, proprietary silicon surface cleaning agent),etc. Substrates may also be cleaned by a combination of such agents. Thesurface of the substrate may be cleaned with organic solvents such asacetone, methanol, trichloroethylene, isopropyl alcohol, etc., forexample, to rid a surface of organic contamination.

In some embodiments, methods may include controlling the residual (e.g.,residual reaction by-products) amounts of dopant in the process chamber.For example, the amount of residual dopant may be reduced by purging thechamber with a gas (e.g., NH₃) while heating to an elevated temperature,prior to introducing the substrate into the chamber. Purging has beenfound to minimize accumulation of reaction-by-products on reactionchamber walls and components.

In certain embodiments in which a diffusion barrier layer is present,the diffusion barrier layer may be formed in-situ with overlying layers(e.g., the III-nitride material region) of the structure. That is, thediffusion barrier layer may be formed during the same deposition step asthe III-nitride material region (e.g., including the optionalIII-nitride material nucleation layer, the optional III-nitride materialtransition layer, the optional III-nitride material buffer layer, and/orthe III-nitride material device region).

The III-nitride material region may be formed using known growthtechniques.

In some embodiments, the optional III-nitride nucleation layer, theoptional III-nitride transition layer, the optional III-nitride bufferlayer, and/or the III-nitride device region are grown using ametalorganic chemical vapor deposition (MOCVD) process. It should beunderstood that other suitable techniques known in the art may also beutilized to deposit these layers including molecular beam epitaxy (MBE),hydride vapor phase epitaxy (HVPE), and the like. In certainembodiments, more than one growth technique may be used to growdifferent III-nitride material layers. For example, in one set ofembodiments, MBE could be used to grow the nucleation layer, and theremaining III-nitride material layers may be formed using MOCVD. Othercombinations are also possible.

Generally, the MOCVD process involves introducing different reactivesource gases (e.g., Al source gases, Ga source gases, N source gases)into the process chamber and providing conditions which promote areaction between the gases to form a layer. The reaction proceeds untila layer of desired thickness is achieved. The composition of the layermay be controlled, as described further below, by several factorsincluding gas composition, gas concentration, and the reactionconditions (e.g., temperature and pressure).

Examples of suitable source gases for MOCVD growth of the optionalIII-nitride material nucleation layer, the optional III-nitride materialtransition layer, the optional III-nitride material buffer layer, and/orthe III-nitride material device region include trimethylaluminum (TMA)or triethylaluminum (TEA) as sources of aluminum; trimethylindium (TMI)or triethylindium (TEI) as sources of indium; trimethylgallium (TMG) ortrimethylgallium (TEG) as sources of gallium; and ammonia (NH₃) as asource of nitrogen. The particular source gas used depends upon thedesired composition of the layers. For example, an aluminum source(e.g., TMA or TEA), a gallium source (TMG or TEG), and a nitrogen sourceare used to deposit films having an Al_(x)Ga_(1-x)N composition.

The flow rates of the source gases, the ratios of the source gases, andthe absolute concentrations of the source gases may be controlled toprovide layers (e.g., transition layers and gallium nitride materialregions) having a desired composition. For the growth of Al_(x)Ga_(1-x)Nlayers, typical TMA flow rates are between about 5 μmol/min and about 50μmol/min with a flow rate of about 20 μmol/min being preferred in somecases; typical TMG flow rates are between about 5 μmol/min and 250μmol/min, with a flow rate of 115 μmol/min being preferred in somecases; and the flow rate of ammonia is typically between about 3 slpm toabout 10 slpm. According to certain embodiments, relatively high flowrates (and also higher gas velocities) can be used, which have beenfound to be particularly effective in minimizing accumulation ofdopants.

According to certain embodiments, the reaction temperatures aregenerally between about 900° C. and about 1200° C. In some embodiments,the process pressures are between about 1 Torr and about 760 Torr. It isto be understood that the process conditions, and in particular the flowrate, are highly dependent on the process system configuration.Typically, smaller throughput systems require less flow than largerthroughput systems.

When forming a compositionally-graded layer (e.g., a compositionallygraded transition layer, which might be formed, for example, withintransition layer 170), process parameters may be suitably adjusted tocontrol the compositional grading. The composition may be graded bychanging the process conditions to favor the growth of particularcompositions. For example, to increase incorporation of gallium in thetransition layer thereby increasing the gallium concentration, the flowrate and/or the concentration of the gallium source (e.g., TMG or TEG)may be increased. Similarly, to increase incorporation of aluminum intothe transition layer thereby increasing the aluminum concentration, theflow rate and/or the concentration of the aluminum source (e.g., TMA orTEA) may be increased. The manner in which the flow rate and/or theconcentration of the source is increased (or decreased) can control themanner in which the composition is graded. In other embodiments, thetemperature and/or pressure is adjusted to favor the growth of aparticular compound. Growth temperatures and pressures favoring theincorporation of gallium into the transition layer differ from thegrowth temperatures and pressures favoring the incorporation of aluminuminto the transition layer. Thus, the composition may be graded bysuitably adjusting temperature and pressure.

When depositing a layer having a constant composition (e.g., atransition layer, a gallium nitride material layer, etc.), however, theprocess parameters can be maintained constant so as to provide a layerhaving a constant composition. When III-nitride material regions (e.g.,gallium nitride material regions) include more than one material layer(e.g., more than one gallium nitride material layer) having differentrespective compositions, the process parameters may be changed at theappropriate time to change the composition of the layer being formed.

It should be understood that all of the layers/regions on the substrate(e.g., the optional III-nitride material nucleation layer, the optionalIII-nitride material transition layer, the optional III-nitride materialbuffer layer, and/or the III-nitride material device region) may begrown in the same process, or respective layers/regions may be grownseparately.

The processes described herein have been described as involving growingthe layers/regions (e.g., the optional III-nitride material nucleationlayer, the optional III-nitride material transition layer, the optionalIII-nitride material buffer layer, and/or the III-nitride materialdevice region) in vertical growth processes. That is, theselayers/regions have been described as being grown in a verticaldirection with respect to underlying layers/regions (including thesubstrate). However, in other embodiments of the invention (not shown),it is possible to grow at least a portion of the layer(s) of theIII-nitride material region (e.g., gallium nitride material layer(s))using a lateral epitaxial overgrowth (LEO) technique, for example, asdescribed in U.S. Pat. No. 6,051,849; or a pendeoepitaxial techniquethat involves growing sidewalls of gallium nitride material posts intotrenches until growth from adjacent sidewalls coalesces to form agallium nitride material region, for example, as described in U.S. Pat.No. 6,265,289. U.S. Pat. No. 7,071,498 entitled “Gallium NitrideMaterial Devices Including an Electrode-Defining Layer and Methods ofForming the Same,” filed Dec. 17, 2003, and issued Jul. 4, 2006, whichis incorporated herein by reference above, further describes techniquesused to grow other layers and features shown in the various embodimentsdescribed herein.

It should also be understood that other processes may be used to formstructures and devices of the present invention as known to those ofordinary skill in the art.

Certain of the layers and/or regions are referred to as being “formedon,” “formed over,” “formed directly on,” “formed directly over,” and/or“covering” another layer or region (e.g., the substrate). It should beunderstood that such phrases include situations in which a top surfaceof an underlying region or layer (e.g., substrate) is converted to thelayer or region that is being formed. Such phrases also refer tosituations in which new layers are formed by depositing the new,separate layer on the top surface of the underlying layer and/or region(e.g., a substrate).

As noted above, the term “region” may refer to one layer or may refer tomultiple layers. It should also be understood that, wherever a singlelayer is described, the single layer may be replaced, according tocertain embodiments, with multiple layers. For example, in certaininstances, single layers described herein can be replaced with multiplelayers that perform a similar function.

The following examples are intended to illustrate certain embodiments ofthe present invention, but do not exemplify the full scope of theinvention.

Example 1

This example describes ion implantation of a species having a relativeatomic mass of less than 5 into a GaN-on-Si HEMT epitaxial structure toreduce the impact of an existing parasitic channel. In this example,protons (H⁺) were used as the implanted species. FIG. 6 is a plot ofcarrier concentration (in cm⁻³) as a function of depth into thesubstrate (in microns), as determined by SRP.

Two similar GaN-on-Si HEMT epitaxial wafers were used in thisdemonstration. The parasitic channel level of the unimplanted wafer isshown as the “control” in FIG. 6. For the “control,” the free carrierconcentration was measured in the middle of the wafer, to characterizeparasitic channel.

The second wafer (shown as the “implanted wafer” in FIG. 6) received aproton implant with an energy of 400 keV and a proton implant dose of10¹⁴ cm⁻². SRP was performed at three locations across this wafer (atthe crown, at the center, and at the flat regions) to assess parasiticchannel level after proton implantation. The effect of the protonimplant on the parasitic channel level was profound. Compared to thecontrol wafer, the proton implant reduced the peak free carrierconcentration by three orders of magnitude. The integrated backgroundcarrier charge was decreased by more than two orders of magnitude. Theproton implant also completely changed the carrier type from p-type ton-type.

Parasitic channel reduction by proton implant was confirmed on more thanone pair of control and implanted wafers.

Example 2

This example describes a set of experiments in which oxygen wasimplanted into a silicon substrate to reduce the free carrierconcentration in the substrate after GaN formation. FIG. 7 is a plot ofcarrier concentration (in cm⁻³) as a function of depth into thesubstrate (in microns), as determined by SRP.

Two silicon wafers comprising similar GaN-on-Si HEMT epitaxialstructures were used. One silicon wafer was implanted with oxygen (O⁺)prior to growth of an epitaxial GaN layer over the substrate. The oxygenimplantation was performed using a 40 keV accelerating voltage and anoxygen implant dose of 5×10¹⁵ cm⁻². The control substrate received nooxygen implant prior to GaN growth. Epitaxial GaN films were depositedon each substrate using the same epitaxial growth conditions.

As shown in FIG. 7, the wafer into which oxygen was implanted prior toGaN growth (shown as “O+ implanted” in the figure) had a parasiticchannel substantially reduced compared to the wafer on which GaN wasgrown without oxygen implantation (shown as “control” in the figure). Inparticular, for the oxygen doped substrate, the peak carrierconcentration was almost two orders of magnitude smaller and the totalintegrated parasitic channel charge was almost three orders of magnitudesmaller, relative to the non-doped substrate.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,and/or methods, if such features, systems, articles, materials, and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A semiconductor structure, comprising: asubstrate comprising silicon; and a III-nitride material region locatedover a surface region of the substrate, a diffusion barrier regionbetween the III-nitride material region and the substrate, wherein: thediffusion barrier region comprises at least one of a rare-earth oxideand/or a rare-earth nitride, the surface region of the substratecomprises a low-conductivity parasitic channel or the substrate is freeof a parasitic channel, and at least a region of the substrate comprisesat least one species having a relative atomic mass of less than 5 at aconcentration of at least about 10¹⁹/cm³.
 2. The semiconductor structureof claim 1, wherein at least a portion of the surface region of thesubstrate comprises at least one species having a relative atomic massof less than 5 at a concentration of at least about 10¹⁹/cm³.
 3. Thesemiconductor structure of claim 1, wherein the substrate comprises atleast a layer having a resistivity of greater than about 10² Ohms-cm. 4.The semiconductor structure of claim 1, wherein the substrate is asilicon substrate.
 5. The semiconductor structure of claim 4, whereinthe substrate is a bulk silicon wafer.
 6. The semiconductor structure ofclaim 1, wherein the III-nitride material region comprises GaN.
 7. Thesemiconductor structure of claim 1, wherein the species having arelative atomic mass of less than 5 comprises hydrogen and/or helium. 8.The semiconductor structure of claim 1, wherein the species having arelative atomic mass of less than 5 is located within the substrate in aspatially defined pattern.
 9. The semiconductor structure of 1, whereinthe surface region of the substrate comprises a low-conductivityparasitic channel.
 10. The semiconductor structure of claim 9, whereinthe low-conductivity parasitic channel has a peak free carrierconcentration that is less than about 10¹⁷/cm³.
 11. The semiconductorstructure of claim 9, wherein the low-conductivity parasitic channel hasa total integrated surface region charge of less than about 10¹²/cm².12. The semiconductor structure of claim 9, wherein the substratecomprises a bulk region below the surface region, the bulk region havinga lower peak free carrier concentration than the surface region.
 13. Thesemiconductor structure of claim 12, wherein the bulk region is dopedwith a first free carrier type and the surface region is doped with asecond free carrier type.
 14. The semiconductor structure of claim 12,wherein the peak free carrier concentration in the bulk region is lessthan about 10¹³/cm³.
 15. The semiconductor structure of claim 1, whereinthe III-nitride material region comprises a III-nitride nucleationlayer.
 16. The semiconductor structure of claim 1, wherein theIII-nitride material region comprises a III-nitride transition layer.17. The semiconductor structure of claim 1, wherein the III-nitridematerial region comprises a III-nitride buffer layer.
 18. Thesemiconductor structure of claim 1, wherein the III-nitride materialregion comprises a III-nitride device region.